There are different options and they depend on the setup, skill...
Let's say the current setup is hot standby HA = 2nd controller has no local APs.
Just turn of the 2nd controller, turn on the new C5210 and configure the HA IP and the major settings should get synced, if all is in sync. move the APs to the C5210, turn off the 1st controller and replace it with the 2nd C5210 and config HA IP and sync again and that is it.
Controllers old/new should have the same software installed.
You'd also change some settings in the config file - mainly the port numbering/naming and import the config file - but for that you need the right skill, if you never worked with the CLI commands just forget it.
-Ron