Clicking on Update Devices will open a confirmation box. If you wish to proceed click on yes.
To send a custom push notification click on the Use Custom Message radial button. The message is limited to 100 charactersand will be broadcasted to all users.
Alternately you can choose the users who receive the message by segment on the right hand side of the pop-up under Limit the users.
Note: If you decide to limit the users to a particular segment, the segments listed are the segments that have changes that need to be pushed out. If there have been no changes since the last update was pushed out for a particular segment, then that segment won't be listed.