You have to Log-In to your OLX account, and update the content of your ad by following the steps below.
STEP 1: Go to Manage Ads tab
STEP 2: Choose the ad that you'd like to edit and click the ellipsis icon
STEP 3: Click the Edit button
STEP 4: Edit the preferred fields and click "Sell your item now" button when you're ready to publish the ad again.