Error: Missing Temporary Folder

This appears to be a common problem with a lot of solutions. I Googled it and found many people have the issue and there are a variety of solutions, all of which require changes to WP code. No idea how to access that code and change it, save it. Can someone tell this non-programmer how to fix this issue with a step x step instruction? I made no changes to my wordpress account but the error just showed up when I tried to download media. Happens with any media and any browser. Thanks for help. This is important.