So here’s the script snippet.

[void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.Office.Server")
[void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint")

#Get any site 
$site = Get-SPSite -Limit 1 
$context = Get-SPServiceContext($site)
$profileManager = New-Object Microsoft.Office.Server.UserProfiles.UserProfileManager($context)
$userProfile = $profileManager.GetUserProfile("domain\UserName")

#Some default properties can be accessed using the dot operator. 
Write-Host ("Changing custom properties for " + $userProfile.DisplayName)

#valid region value 
$userProfile["CpSingle"].Value = "Asia"

#First clear any existing item for multi property. Then add our values. $userProfile["CpMulti"].Clear() 
$userProfile["CpMulti"].Add("India")
$userProfile["CpMulti"].Add("Bhutan")
$userProfile["CpMulti"].Add("Sri Lanka")

#Finally save all the changes.
$userProfile.Commit()

This script is good for modifying the custom metadata properties of a single user. But if we want to make these changes or some other changes uniform to all the existing UserProfiles, then we need to enumerate through all the userProfiles instead of fetching a single value. Same script to update the above mentioned custom properties with the same value for all the UserProfiles is mentioned below.

$profiles = $profileManager.GetEnumerator()
while ($profiles.MoveNext()) {
     
     #Get the current profile
     $userProfile = $profiles.Current     
     Write-Host ("Changing custom properties for " + $userProfile.DisplayName)

     #valid region value 
     $userProfile["CpSingle"].Value = "Asia"

     #First clear any existing item. Then add our values. 
     $userProfile["CpMulti"].Clear()
     $userProfile["CpMulti"].Add("India")
     $userProfile["CpMulti"].Add("Bhutan")
     $userProfile["CpMulti"].Add("Sri Lanka")


     #Finally save all the changes.
     $userProfile.Commit()
}