...more than one way...
...method one is using the transform tool, click on a corner bottom left or right then press and hold command+alt/option+shift keys and drag inwards...
...method two, choose effect > warp > arc, turn the bend value to zero and adjust the vertical value to a negative, positive value will do the top edge. Now when using the effects options, these are live effects which means they are adjustable further down the road if needed...
...to change a live effect of an object, use the appearance palette and double click the effect and it will allow you to modify the settings again, this is true of all those options under the 'effects' menu...
...to go beyond live effects you need to expand the appearance, found under the object menu...
...method three is to create a rectangle on top of your target objects (grouped), adjust the corner points mathematically or manually using the direct select tool, select both the target object and the top adjusted rectangle...
...go to object > envelope distort > make with top object...
...envelope distortions can be any shape, and is also a way of distorting imported bitmap graphics...
Andrew