Navigate to your "My Projects" page from the options at the top of the screen.

Select the "Sent Proposals" tab to find all of the proposals you've sent that haven't yet been accepted or declined. (These will all be labeled with the "Pending" status.)

Click the banner of the proposal that you're interested in editing.

If you scroll down, you'll see the content of the proposal under the "Proposal Sent" tab. Click the blue pencil icon in the right corner there to edit.