There should be no problem when updating your theme if you haven’t made any changes. However if you want to be sure you can unpack the theme zip and rename the theme folder. For example, rename it to “therestautant53”. Next rezip the file and upload it to WordPress. This way the new version of the theme will be installed next to the old one and you can go back to the old version of something went wrong.

Always make a backup before you do anything, just to be sure 🙂