I don't live near my family. I am scared. My father's memory problem are new. How do I talk to him when he thinks in the past?

If dad lives with someone, or another relative is close that might alleviate some problems.
If dad is married, or living with someone they are the ones that will need support. Ask how they are doing.
If dad is living on his own take this time when you visit to assess how he is doing and discuss with him your concerns and feel out the possibility that he would consider Assisted Living. (As he declines there will be a transition to Memory Care.)
You also need to ask him what his plans are if something should happen to him.
You should also probably make a visit to an Elder Care Attorney if he has nothing in writing, no plans, and no support.
Is dad a Veteran? If so a call to the VA or the local Veterans Assistance Commission office can get you started to determine what benefits he qualifies for.
You also need to determine what type of role you are going to play.
Can you care for him? If so would he be willing to move or will you relocate??
Can he afford caregivers? If so full time or part time?

Break down the discussions a little bit at a time so he is not overwhelmed and frightened. Also give him plenty of time to respond to your questions. (It can take someone with dementia 45 seconds, or more to respond to a question. For me 45 to 60 seconds when waiting for an answer is a LIFETIME!)

Most important...enjoy this visit try not to put pressure on him or yourself. Do things with him that he has enjoyed in the past.

And some memory problems can be a result of depression and isolation. Having lost a sibling and the past year + with COVID may have had an impact.

Is there other family or support where your Dad lives?
Sounds as though you need to arrange and assessment while you are visiting. I wish you luck.

Does he live alone? Are there other family members near, who might interact with him often?

It is VERY easy to miss the early signs of memory loss. This is especially true when interfacing isn't frequent or long, such as during phone calls. Many PCPs miss these as well, since they don't see the patient very often and it's only for a short while (we spend more time with the office staff and nurses who check vitals, etc!)

Many people can *appear* relatively normal during the early stages. When my mother started repeating herself, I realized something was wrong and started my research - I knew nothing about dementia at that time! She lived alone, so some issues weren't noticeable until changes were made. For instance, once we took the car away and I had to help by delivering supplies and/or taking her grocery shopping, it became evident that she was no longer cooking. Those fresh veggies bought last time were all shriveled up in the fridge. She was relying on frozen dinners and boxed stuff.

Even later, after we moved her to MC, I realized there were even more subtle warnings before that. Not knowing anything about dementia, I dismissed the issues as nothing but mom complaining. These were 1) saying someone she had in to paint was taking her broken jewelry she'd set aside to "cash in" and 2) accusing OB of taking her tweezers when they stayed there one time. Really mom? Tweezers? Why? They are like $2!! I bought her another to calm her down. When clearing out her stuff, I found THREE in the bathroom drawer and another 5-6 in a plastic container in her dresser drawer!

That said, there ARE other treatable conditions that can mimic dementia and/or cognitive declines. UTIs. Imbalances in the blood. Dehydration AND Overhydration. I don't know the names of other conditions, but many are treatable and resolve the memory issues.

Is it possible to get him to his doc for a full checkup, including blood and urine tests while you are there? Many docs now include a minimal test (really minimal) that is actually meant to establish a baseline and then retest watching for changes in ability, but this test isn't complete or conclusive! If the checkup and tests are all negative, it might take referral to a specialist, to determine what the underlying cause is and get instructions/information on where to go from there.

Has he assigned POAs, written a will and medical directives, have assets that need protection (from him and other nefarious peoples!)? If not, this is something else you and family need to pursue, the sooner the better. Even in early stages, an atty will take him aside, ask questions and determine if he is still capable of signing these documents. We had most already done, but had to set up a trust, so the atty did question mom and decide she was still capable of understanding what we were doing.

Prepare for the worst, hope for the best! Being proactive at this point is your best plan for dealing with your concerns.

If your dad is indeed only 51 and has memory issues where he thinks the present is the past, he needs a thorough check-up immediately.

Your profile says that your dad is only 51. That's awful young for "memory" problems. Could there possibly be something else going on?
If in fact it is memory issues, you just go along with him in the conversations. You don't argue and tell him he's wrong or that something isn't true, you just go along with whatever he says. He's still your dad, and I'm guessing he still has most of his cognitive abilities in tact, so go and enjoy your time with your dad.