So you've created an Asynchronous Governance Proposal and now want to withdraw it from consideration.
You can do it from your Tensions page, where you probably added the Proposal in the first place.
From that page, you can click on the Tension associated with the Proposal. You have two choices: withdrawing the proposal (which keeps it saved as a draft), and outright deleting the tension. Deleting the tension will automatically delete the attached proposal as well.
If you withdraw the proposal, you will later be able to delete it without deleting the tension, by clicking on "Proposal Drafted" and then clicking the trash icon on the window that pops up.