You will have to login in on the web app using your same email/password credentials.
When you signed up for treezi, you would have been given a unique URL. Something like: yourcompanyname.treeziapp.com
Once you are logged in:
1. Go to the "Proposals" tab.
2. Find the Proposal you want to delete and click on the "X" in the far right.
3. You can also click on the drop-down to change the status of the proposal.
4. Click on the Proposal ID to open the proposal to edit further.