Start by going to your proposal
Click Sales --> Proposals to locate the proposal, or search for the title, client name or invoice number in the global search bar at the top center of your account.
To change the deposit amounts on the proposal alternatives, go the invoice tab of one of the alternatives and click the 3 dot menu next to the deposit line.
On the deposit edit screen, choose whether to adjust the deposit for all proposal alternatives, or just one of them.
