For single user you can use below script

Example

$Mobile = "+1234567890"
Set-ADUser -Identity Joe.Doe -replace @{ 'MobilePhone' = $($Mobile) }

Copy and Try it

for all users - want to capture city and country from any valid user and update for all users,
you can create list and load list of users to update their properties
I want to update city and country only

Example

#any valid user whose property you want to apply all users
$username = "anyvalidADuser"

#fetch properties example, here i want to capture city and country -- AD field name for city is 'l', country 'co' 
$userprops = @{
        country = get-aduser $username -Properties co | ForEach-Object {$_.co};
        City = get-aduser $username -Properties l | ForEach-Object {$_.l};
    }

$users = Get-ADUser  -Filter * -SearchBase $ou |Select-Object SamAccountName | ForEach-Object {$_.SamAccountName}

foreach ($user in $users)
{
    set-aduser -identity $user -Replace @{
    co = $userprops.country;
    l = $userprops.city;
    }
}

Copy and Try it