It's not possible to edit a proposal once it has been accepted by your client. At this point, it's a legally binding document and cannot be edited.
However, if amendments need to be made because the terms have changed, or your client did not select an option in your pricing table for example, a new proposal will need to be created and sent to your client.
The quickest way to do this is to navigate to your proposal and click Options > Save as Template and create a new proposal from this template. Make any changes that are necessary and resend to your client.
If your client did not select an option from your pricing table, it might be worth adding an extra line above your pricing table to make it super clear they need to select the products/service they wish to purchase. You could even go one step further and add a final nudge above your signature block to make sure they have made their choices in the Investment/Pricing section of your proposal.
In terms of the original proposal, this cannot be deleted as both parties legally must maintain access to this proposal. However, you can "Hide" this proposal which will remove the proposal value and data from your reporting so you won't have multiple versions being tracked.