On the Gmail app for iPhone or iPad, you want to select the sender’s image or letter - they’re typically in the shape of a circle and a checkmark will appear. You then manually select all of the ones you wish to delete and then hit the trash bin again.
For the Android app, you will want to click on the menu in the top-left corner that has three horizontal lines (also known as the menu burger). By selecting it, you will see the list of your inboxes, including promotions, which you will select. As with the iPhone app, you then do a long press and select each email you wish to delete, before hitting the trash bin.