How can I change her diapers in bed without hurting her?

I agree with the answer above, however, if she can't roll herself consider using a draw sheet. You fold a flat sheet in half and put in on the bed so that she lays on it. The top of the sheet will touch her shoulders. You then pull the sheet (gently) to move her from side to side or pull upwards to roll her. This helps prevent skin tears etc.

It's easiest with a hospital bed. You won't have to bend down so far, and there are bars so she can hold on. First lift the bed as far as it will go. Have her lying flat on her back. Take old diaper off wash area and put on some ointment if necessary. Next tell her to roll on her side, and hold onto the bar so she won't roll back. Place new diaper under her as far as it will go, then tell her to roll onto other side, and hold onto the other bar. pull diaper out, it shud now be plush under her. Have her roll back onto her back and just fasten it in place. That was the easiest way for me to put diapers on my mom. Good luck...Noticed the nursing home does it the same way....:)