Going Back to WP Version 3.5.1

Can anyone please help me get back to using WP 3.5.1?

I am currently using Version 3.6.1. After only a couple months of use, I discovered that my theme is not compatible with any version of WP newer than 3.5.1. According to the “theme-check” plugin, my beloved theme has a raft of compatibility issues. Not being a programmer, I don’t know if this list of roughly 50 issues is easy to resolve or not. To date, the theme’s developer will not resolve these issues/refuses to respond to emails/etc. after numerous attempts to reach him. I know people have cautioned about using older versions of WP, but no one has outlined specifically what those risks are. At this point I am willing to take the risk, because I’ve put in too much work designing my site with this theme to turn back now.

However, I am worried that I will lose my entire website if I do not properly reinstall (?) WP 3.5.1. I don’t want to do anything wrong (e.g., back up my site incorrectly) as I attempt to do so.

Can someone please offer a step-by-step way to get back to WP 3.5.1 from 3.6.1, one that is easy for a relative novice to understand? Thank you!