You can easily mark the gift as purchased yourself. Simply login to your account and hover over the item you would like updated to click the pencil icon. The option to Mark As Purchased will be located at the bottom of the panel that opens. Simply select this option to update that gift as received on your registry.
**Please note, items that are Synced must be marked as purchased on the store's website first. MyRegistry.com will then mirror the change made to your synced account.