Hey @Janak
Try this:
$Users = Get-ADUser -Filter {UserPrincipalName -like '*YOURCURRENTUPN.local'} -SearchBase ",OU=EXAMPLE,DC=YOURDOMAIN,DC=local" -Properties UserPrincipalName -ResultSetSize $null
$Users | foreach {$newUpn = $_.UserPrincipalName.Replace("YOURCURRENTUPN.local","NEWUPN.com"); $_ | Set-ADUser -UserPrincipalName $newUpn}