Bulk update the Department field in active directory using Powershell Get-ADUser and Set-ADUser cmdlets.

# define the OU you want to set
$ou = "OU=Oregon,OU=Sales,OU=Users,DC=contoso,DC=com"
# define the server you want to make the changes on
$domainController = "dc1.constoso.com"
# set department text
$departmentText = 'Oregon - Sales'
# get the list of users
$users = Get-ADUser -Server $domainController -SearchBase $ou -Filter {(ObjectClass -eq "user")} -Properties Department
# apply the new department to the users we found
ForEach ($u in $users) {
    $u | Set-ADUser -Department $departmentText -Server $domainController