Remove the screws from the sheetmetal cover at the top of the door. The glass can then be removed thru the opening revealed thru the top of the door. The bottom channel that the glass fits in (it is originally held in that channel with a rubber/cork glass setting material) can be unhooked from the regulator and removed with the glass still installed. Another method would be to slide the glass out of that lower channel (it can be very tight) and then slide just the glass out of the opening at the top of the door. The glass does NOT have to be broken to be removed.
The replacement glass can either be installed into the lower channel and then slid down the window tracks thru the top opening in the door and then re-connected to the regulator arm or attach the channel to the regulator arm first. Then slide the glass down the window tracks thru the opening at the top of the door. Getting the glass into the bottom channel can be frustrating as the rubber/cork setting material provides considerable interference between the channel and the glass. Lubrication and tapping straight down on the glass can assist setting the glass in the bottom channel.
Before installing the new glass, check the condition of the front and rear window channels. They may need replacing and are available at most Model A Ford parts distributors. Bratton's, Mac's, Snyder's and Mike's A-Ford-Able
are all good sources for parts.