In most cases, you will only need to worry about your UBIT password. To change it, go to the UBIT Password Management page and follow the instructions there.
There are very few systems for which SENS maintains separate passwords. You will use the same UBITname, but the password will not necessarily be the same as your UBIT password (there's nothing to prevent you from setting them to the same password, however). If you need assistance changing your password on one of those systems, please contact SENShelp.