To convert proposal to invoice or estimate, select the proposal from the table and on the right side a convert button will be shown.
Click on the button and choose Estimate or Invoice. After you choose, a popup will be shown with the preview info and also the parsed items which are auto inserted to the estimate/invoice.
When you are satisfied from the result, click Save.
After converting, the proposal will be auto marked as accepted (if is not already marked as Accepted).
If the proposal is related to some leads which are not converted to the client, you will need to first convert the lead to client, and then get back here and convert the proposal to estimate or invoice.