If you would just like to change your password and know your existing password, loginto your Moodle account as usual.
Go to the administration block, under My profile, click change password. (Note: some institutions use an authentication method that does not store your passwords in Moodle. If you do not see the change password link, you will need to contact the IT department for your Moodle site)
On the change password page, fill in your old password, the fill in your new password and retype it. Click Save Canges when done.