Sadly locating all the files on the device can be complicated to say the least. The simplest and most straight forward way is to save your personal stuff (contact, photos, videos, music, etc.) to a computer or cloud account (be sure you save any stuff within apps as well like bookmarks in the browser, etc.) and then do a "Hard Reset" of the phone. This will wipe anything and everything on the device, and then start it fresh.
NOTE: Doing this will clear 'all' the data on your phone, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the phone.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the phone...