On Thesis main website there is video explaining how to upgrade.
1. Back up the /custom folder from your existing Thesis installation to your hard drive. If you're upgrading from Thesis 1.5.1 or older, you'll need to place your /rotator folder and your layout.css file inside your /custom folder.
2. Upload the new Thesis 1.7 folder (/thesis_17) to your /wp-content/themes folder.
3. If you're upgrading from Thesis 1.6, simply upload your backed-up /custom folder to your new /thesis_17 folder, and then skip to Step 5. If you're upgrading from a version of Thesis prior to 1.6, rename the /custom-sample folder to /custom, and do not overwrite this folder with your existing /custom folder! Then proceed to
Step 4.
(This step only applies if you're upgrading from a version of Thesis prior to 1.6) Instead of overwriting the new /custom folder with the one you just backed up (as you are probably accustomed to doing with previous versions), drag the contents of the /custom folder you backed up in Step 1, and drop it all into your new /custom folder, which should also contain layout.css.
5. Set the permissions of your new layout.css file to 666.
Make sure your /cache folder's permissions are set to 775.
6. Inside your WordPress dashboard, go to Appearance → Themes, and then activate Thesis 1.7.
7. Visit the Design Options page from the new Thesis links on the left side of your WordPress dashboard, ensure that Thesis is not giving you a warning about the file permissions on your layout.css file, and then hit the Big Ass Save Button.