If you mean how to access the new addon features, I recently had to go through the same process and found no documentation anywhere to help a person new to WordPress and plugins. I managed to figure it out….
Install purchased addon plugin:
1. From Purchase Receipt email, download the zipped plugin file and save on your computer. Don’t bother unzipping it.
2. From the WordPress Plugins page, Add New.
3. Click Upload at the top, browse and select the saved, zipped file.
4. Activate the plugin, if not automatically done.
To activate License Key:
1. Go to Appearance-GeneratePress page.
2. Enter in key from Purchase Receipt, into License Key box and save.
The new Colors addon options can now be found on the Appearance-Customize page.