Title says it all, here is the script,
1: $csvfile="users.csv"
2: $mySiteUrl = "enteryourmysiteurlhere"
3: $upAttribute = "attributeToUpdate"
4: $site = Get-SPSite $mySiteUrl
5: $context = Get-SPServiceContext $site
6: $profileManager = New-Object Microsoft.Office.Server.UserProfiles.UserProfileManager($context)
7: $csvData = Import-Csv $csvfile
8: foreach ($line in $csvData)
9: { 10: if ($profileManager.UserExists($line.username))
11: { 12: $up = $profileManager.GetUserProfile($line.username)
13: $up[$upAttribute].Value = $line.attributeval
14: $up.Commit()
15: }
16: else
17: { 18: write-host $line.username " profile not found"
19: }
20: }
21: $site.Dispose()