Just by comparing your current version with that original theme version so you can see what was changed, you have to use a software like Kdiff.
My advice:
1) Unzip the latest Atelier files to your computer and change the main theme folder from Atelier to Atelier_new
2)Then upload the Atelier_new to the wp-content/themes
3) After this you just need to switch the folder names to activate the last version, rename the current Atelier directory on the server to Atelier_old and rename Atelier_new to Atelier.
If you need to revert just change the directory names again.
-Rui