Hi Friend,
You need to keep the following points in the mind.
First thing you have keep in mind is downtime. once you upgrade the controller all APs will up download the updated code from the controller and reboot.
As a good practice,
1. You can try "AP Image preload" feature to reduce the AP downtime or
2. You have to change the LMS of all the APs to the Local and then upgrade the Master controller.
2. Terminate only one AP on the Master after the upgrade and test the connectivity and then terminate other APs accordingly.
3. If you want you can enable centralised Image management on the Master so that it will upgrade the image of the Local otherwise upgrade the Local manually .
Hope you got some clarity on this.
Please feel free for any further help on this.