You can easily revise a pending proposal that you've sent to the lead. To do this, go to:
Shoots > Pending > Open Proposals
Find the shoot whose proposal you want to modify and click on it. Then go to the section:
Correspondence > Proposals
You can then click on the Revise link or you can click on the pencil icon to edit the proposal.
A pop-up dialog will ask you if you want to Cancel the Proposal and Revise it. You want to click "Yes". All the previous details you had originally input will be preserved, but it will just take you through the proposal creation steps and allow you to modify any parts that you wanted to change. You'll then need to email the revised proposal to the client.