Either way will do but if you have Local controllers deployed, as a best practice upgrade the Standby first and then Master.
1. Upgrade the image of backup partition of Standby
2. Upgrade the Image of Backup partition of Master
3. Enable ” AP Image Preload” if you want to reduce the down time
4. Reload both the controllers.
Note : don’t forget to take the flash back up of the master before doing the above.