Can I make changes on a Guardianship and Power of Attorney on my own or do I need an attorney?

CONTINUED..... Sister is not willing to sign off on guardianship! She has been dad's guardian for almost 5 years, but has never filled out any of the required yearly forms, nor has any contact with dad at all. My question is in need of an expert, Can she prevent this from happening for dad, & how hard can she make this!?
Please HELP!!

Thank you both for the answers. I have retained an attorney, as for the situation is becoming difficult! My sister is Dad's Guardian. She lives in the same town, but has no involvement & has not spoken or seen Dad in over 6 months! Yet, she claims she has full control over Dad & his decisions!! Dad's Doctor has written," he is capable of making his own decisions & does not need a Guardian." We are hopng she will sign off on the Guardianship papers without any troubles!?
I take care of my Dad, he lives with me. Should I have POA? I'm concerned, if something happens & he needs medical care!? Any advice?
Thank You

You need an attorney. Only an attorney or a paralegal under the direction of an attorney can change a POA and definitely a guardianship rule.

Make your changes or new POA/etc. and execute them in front of two Witnesses. Depends on your state, but this should suffice.