Im sure there are guides out there for helping with reverting to a previous wordpress version. Wordpress like all CMS , they tend to be pretty easy to redo individual pieces. Open the wordpress editor thingy, and im sure the error would stand out. If not, remove the menu and just re-add it.
OHHHH i think i know whats happening. When you updated, it probably updated bootstrap or whatever CSS styling that was done for your menu. What you are seeing is additional stuff that swaps out between a smaller resolution \ mobile phone kinda thing. With that said... you might be able to change the version of bootstrap or whatever is styling your menu in the wordpress configs.