@matt, your theme is in wp-content/themes/<yourthemename>
. So long as you don’t overwrite that, your theme is safe. You should back it up though. Its just good habit. What you need to upload is in /wp-admin
— not even in the wp-contents
folder much less the wp-contents/themes
one. To upload it, you need to…
1) login via FTP (my preference) or find your host’s file manager if there is one
2) browse to your local copy of WordPress
3) find the wp-admin folder, select it, but don’t browse into it. You want to upload the whole folder.
4) Now browse the remote server until you see the wp-admin folder. Don’t browse into that folder though.
5) Now upload the wp-admin folder you selected previously in step 3 Most FTP has a fairly obvious ‘upload’ button.
5) If you screw something up, don’t worry about it because you backed up your theme already anyway, right? Just start over, deleting whatever it was you put in the wrong place.