Modify an user attribute using Powershell script

Question

In the current account creation script I am setting the following two attributes targetAddress and proxyAddressess:
$UserAttributes[0].Add('targetAddress', $User.mail)
 $UserAttributes[0].Add('proxyAddresses', "SMTP:" + $User.mail)
I need to be able to change these values sometimes.  I thought if I replaced add with replace that would work it did not.
Ideas?
Here is the whole function:
 $AttributeItems=$UserAttributes[0].GetEnumerator()
 ForEach ($AttributeItem in $AttributeItems) {
  $AIKey=$AttributeItem.Key
  $NewAttribute=$AttributeItem.Value
  $ADAttribute=$FullUser.$AIKey
  if ($ADAttribute -ne $NewAttribute) {
   Set-ADUser $User.sAMAccountName -Replace @{$AIKey=$NewAttribute}
 $UserAttributes[0].Add('targetAddress', $User.mail)
 $UserAttributes[0].Add('proxyAddresses', "SMTP:" + $User.mail)
 $ClearCount=$UserAttributes[1].Count
 If ($UserAttributes[1].Count -gt 0) {
  Set-ADUser $User.sAMAccountName -Clear $UserAttributes[1]
 $enattendantgodot='Hold on there pardner!'

Answer 1

I'm using example 4.
This is what I have: 
$UpdateUserAttributesArray = 'sn','givenName','initials','displayName','info','mail','departmentNumber','department','telephoneNumber','ipPhone','title','physicalDeliveryOfficeName','employeeType'
Function StudentNameChange {
    $Attributes=BuildUserTable $UpdateUserAttributesArray
    Set-ADuser $User.sAMAccountName -Replace @{mail="$User.mail";targetAddress="$User.mail";proxyAddressess="SMTP:$User.mail";sn="$User.sn";cn="$User.sn + $User.givename"}
$UpdateUserAttributesArray sets the value for the import file.
StudentNameChange should set the updated value has needed.
The script runs but no settings are changed.
Set-AdUser $User.sAMAccountName  -mail $User.mail -surname $User.sn
Like that.,
Replace is only for optional attribute.
Function does not specifi $user so that may also be missing.
I think you need to spend some time learning the basics of PowerShell. It looks like you are just gluing things together thazt you have found with no understranding of how any of the bits work.
Try just setting one user at a command prompt until you see how the CmdLet and PowerShell work.  Post single simple questions as you try each parameter.
# build a test user object
$user New-Object PsObject@{
SamAccoutnName='somename'
Mail='[email protected]'
SN='lastname'
GivenName='John'
proxyAddresses='...<list of addresses>'
#start like this and add items until you understand what is working.
Get-AdUser $user.SamAccountName |
Set-AdUser -Surname $user.sn -GivenName $user .GivenName -Replace @{Mail='$($user.Mail'}
#use Rename-Object to change CN.
You need to learn the simple bits before you try to do grand things.
¯\_(ツ)_/¯