There are two different set of commands that may work if you follow the following process:
Option # 1:
1- Close your RMAD console.
2- Open PowerShell and invoke RMAD Shell Management with Add-PSSnapin Quest.RecoveryManager.AD.*
3- Or if you prefer, you may open RMAD Shell Management console directly by going to Programs > Quest > Management Shell.
4- Run the following commands replacing "TheNameOfYourCollection" with the computer collection name in which you want to modify agent credentials: $cred=Get-Credential
$a=Get-RMADCollection -Name "TheNameOfYourCollection"
Set-RMADCollection -InputObject $a -Properties @{AgentCredential = $cred}
5- After you run the above a screen will pop up asking to enter credentials. You may enter the credentials that you want to add in the agent tab.
6- Open RMAD console and validate that the correct account is set under agent tab.
Option # 2:
The only differences between both methods are that Option # 1 does not replace credentials under agent tab if the credentials are empty.
Option # 2 does not open a pop up window asking to enter credentials.
If you don't want to close and reopen the console while running any of the above commands, then make sure to refresh the Computer Collections root folder to make sure the credentials get updated correctly. If you only refresh the specific computer collection that you are modifying and not the Computer Collections root, then it will not show the new credentials under Agent tab.