Here’s a deceptively effective exercise for reversing a slumped upper back. Don’t be tempted to use too large a roll or stay on it for too long (30 seconds is a reasonable place to start). You can slowly build up the roll and duration as your back becomes more flexible (this can take 6-8 weeks).
If you can’t find a place to lay down, it can be done standing against a door or wall.
In both versions of the exercise, you can move the roll further towards either your head or lower back.