There are two ways to update the financing method on a proposal, this guide will walk through both ways.
From the proposal stage, click "Edit"
Next, click the Financing Method dropdown and change the Financier.
Once the financier is updated, select the new financing product.
On the Finance Terms slide inside of the proposal, click the lender name. Select a new financing method and product.
Enter in a down payment if needed and click "Save".