Niki from Trulia Customer Support here. We are excited you are using Trulia for your real estate needs.
You can edit your listing by logging in to Trulia, hover your mouse pointer over your name in the top right corner of the page. The site navigation menu will appear as a drop down menu, click on "my listings". Once in the listings page you can click on the blue text next to the "open house" field or your property will appear with an "edit property" button and you can go to the "open house section here. Please note that the estimated time for the edits to go live is 15 minutes, however depending on the data it can take up to 24 hours to show live on the listing.
Please feel free to contact us directly at firstname.lastname@example.org, or by phone 1-888-466-3501, Monday - Friday 7am to 4pm PST.