If you have been already granted a TSS visa and you want to change your employer, you are not required to apply for a new visa. Before you start working for a new employer, however, you must be nominated by your proposed new employer (who must also be an approved business sponsor) and have that nomination approved first.
If you start working for your proposed new employer before the new nomination is approved, you will be in breach of visa condition 8607 and your TSS visa might be cancelled.
The approval of a new nomination only allows you to change your employer. It does not extend the length of your TSS visa, it does not allow you to work in a different occupation and does not change the conditions attached to your visa.