Need to change current Backup to Master and Master to Backup
On a chassis with 2 MSM/MM's you can change the Master to the other MSM/MM with the command run failover. Check if backup MSM/MM is (in sync) using command show switch. If backup is in sync do the failover, this caused the current master MSM to reboot and the current backup to become master. This is in most cases hitless.
On a stack you can do a failover which will cause the current backup to become Master and a new backup is selected out of the available master-capable stack members. When you do a failover the current master slot will reboot causing all ports on that master slot to go down until the slot comes back. If the stack consists of more than 2 master-capable slots the new backup can be a different slot than the just rebooted slot.
show stacking detail
In most cases there is no need to change the master, only when a stack consists of different switches you could change master to have the most powerful CPU/memory switch master.