I just upgraded to Ubuntu 17.04 and I'm having some problems that I didn't have before, so I would want to know if it's possible to restore Ubuntu into 16.10 without losing files and apps.
I'm sorry, you should backup all your files and go for a clean install of Ubuntu 16.04 LTS. If you're looking for a stable system, don't try the .10 or any non LTS versions unless you're using new hardware (that was just released). Unfortunately, Ubuntu doesn't do downgrades.
If your hard disk is large, I suggest you create a separate smaller partition where you will place a backup of all your files, and then install Ubuntu in the remaining portion of it. This makes data transfer much faster than using external media.