Update value of CMIInstance ApplicationPoolIdentity Password value

I am able to get to a Username value of the Application pool identity via below script

$tmp = Get-CimInstance -Namespace root/MicrosoftIISv2 -computer $servername -ClassName IIsApplicationPoolSetting -Property Name, WAMUserName, WAMUserPass | select Name, WAMUserName, WAMUserPass

write-host "AdminACLBin:" $tmp[2].WAMUserName

I want to update the WAMUserName to something else. What should be the script for this? put() doesn't seem to work for CIMInstance. Please help..