What version you currently have installed ?
What version are you planning to upgrade to ?
Make sure you have a copy of the backupflash
1-make sure you have enough space delete any unused files : crash.tar , logs.tar, backupflash
2-you can load it on the unused partition
3-if you have LMS primary/backup , remove the backup IP address and added back after the upgrade has been completed
4-since the code is in the unused partition once reboot the controller will come up on the version
5- you should reboot all the controllers at the same time