Reinstall OS without losing personal files

[AC Question]

My phone has started to do its own thing: hang up when I'm in the middle of a conversation, enlarge the screen so that I can't see all of it, refuse to respond to a swipe to open, etc.What's the best way to save my personal information, reinstall the OS (Android 5.0.1) and then add back my personal information such as contacts, etc.

 

[belodion]

If you mean Factory Reset, the guides linked here will help:
how can I reset my phone
http://forums.androidcentral.com/showthread.php?t=624636