To select multiple messages to take action on at one time:
1. Tap on the avatar picture to select messages. Messages that are currently selected will show a check mark.
Once your messages are selected, you can take action on them two ways:
1. Tap on the action grid button in the top right corner.
2. Select the action you would like to take.
1. Begin swiping one of the selected messages to the left or right.
2. Release the swipe to take the swipe action on all messages.
After tapping on an avatar, you will also notice the ability to "select all" or "select all from sender" at the bottom. Tap on "select all" to take an action on all messages. Tap on "All (sender)" to take action on just those messages from a particular sender.