1) Use GET_AD_Ext_Attrib2
Save a new file with only usernames of users with some data in extensionAttribute2 
CSV with “SamAccountName of users with extensionAttribute2 set previously
   userexport_ext2_toclear.csv


2) 


TEST
$users = Import-Csv -Path C:\Autotask\AD\Tech1_to_AD\userexport_ext2_toclear.csv

foreach ($user in $users) {
Get-ADUser -Filter “SamAccountName -eq '$($user.sAMAccountName)'” -Properties * 
}


UPDATE

$users = Import-Csv -Path C:\Autotask\AD\Tech1_to_AD\userexport_ext2_toclear.csv
# Loop through CSV and update users if the exist in CVS file

foreach ($user in $users) {
Get-ADUser -Filter “SamAccountName -eq '$($user.sAMAccountName)'” -Properties * | Set-ADUser -Clear extensionAttribute2
}