param (
    [string]$oldPassword = $( Read-Host "Old password"),
    [string]$newPassword = $( Read-Host "New password")
)

$ADSystemInfo = New-Object -ComObject ADSystemInfo
$type = $ADSystemInfo.GetType()
$user = [ADSI] "LDAP://$($type.InvokeMember('UserName', 'GetProperty', $null, $ADSystemInfo, $null))"
$user.ChangePassword( $oldPassword, $newPassword)

param (
    [string]$oldPassword = $( Read-Host "Old password"),
    [string]$newPassword = $( Read-Host "New password")
)

$MethodDefinition = @'
[DllImport("netapi32.dll", CharSet = CharSet.Unicode)]
public static extern bool NetUserChangePassword(string domainname, string username, string oldPassword, string newPassword);
'@

$NetAPI32 = Add-Type -MemberDefinition $MethodDefinition -Name 'NetAPI32' -Namespace 'Win32' -PassThru

$NetAPI32::NetUserChangePassword('.', $env:username, $oldPassword, $newPassword)

([ADSI]'LDAP://CN=User,CN=Users,DC=domain').ChangePassword('currentpassword','newpassword')

$MethodDefinition = @'
[DllImport("netapi32.dll", CharSet = CharSet.Unicode)]
public static extern ***long*** NetUserChangePassword(string domainname, string username, string oldPassword, string newPassword);
'@