When upgrading to 3.7.5.3
You may need to make a slight alteration to your theme. Please see the Theme Notes section of this post.

When upgrading within 3.7 itself (for example 3.7.5 to 3.7.5.3)

1. If your wp e-commerce theme is still located at wp-content/plugins/wp-e-commerce/themes/ you should move it to wp-content/uploads/wpsc/themes/ in order to preserve your changes
2. De-activate the old wp e-commerce plugin
3. Upload the new wp e-commerce plugin
4. Go to the plugins page and activate
5. Ta Da!

When upgrading to 3.7 from any older 3.6 version

1. De-activate the old wp e-commerce plugin
2. Upload the new wp e-commerce plugin
3. Go to the plugins page and activate
4. Once activated, go to your purchase logs
5. You will see a red message box on the top of the page asking you to update your purchase logs
6. Click on the link and follow the instructions
7. Check that all of your settings are still intact
8. You are ready to roll!

Technically, if you are tech-savvy, you might be able to get away with going to the plugins tracker and figuring out exactly which files have changed, and only upload those files (instead of the whole plugin). However, you do run the risk of missing something, so its usually best to just upload the whole thing.

Note: Auto Updates from WordPress are disabled in current versions of WP e-Commerce and will stay disabled until WordPress offers plugin developers (like us) the ability to protect your modified files. If you are using a version of WP e-Commerce that does not have Auto Updates disabled you should do a manual update to the latest version to preserve your files and modifications.