This article explains how to amend default partners for cases in Accounts within Unity® Practice Management. 

When opening a new case in Unity:

The default partner should always be visible:

If there is no default part listed OR if any amendments need to be made to add/remove partners, cashiers can login to the Accounts section of their Tenant (Via RDP) Click on Maintenance:

Then search for the partner in the list and tick/untick the Partner option (depending on if you want to add/remove a partner from matters: