Concealed closers 101-
The door you describe probably uses either a concealed header closer (inside the header frame) or a concealed floor closer. A header closer usually has a removable plate to access the 2 adjusting screws. A floor closer some times has 2 small holes right in the threshold for access to the adjustments but sometimes you need to remove the threshold (or part of the threshold) to access them. Open the door and look up or down right near the pivot (spindle) area.
If you see a single, slotted screw in the header close to the pivot area that IS NOT THE ADJUSTMENT. That is the way you retract the top pivot on some doors that use a floor closer. Dont turn that screw or you may drop the door on yourself. Netsearch "dorma walking beam" or to see an example of what I mean.
Once you get to the 2 adjustments... usually, gradually tightening one screw will slow the travel from approx 120deg to about 20deg or so. The other screw usually controls the latch speed (0-20deg +-).
SOME concealed closers dont work this way though. In those cases, one screw controls the entire swing and the other screw is a hold-open feature (holding the door open for delivery persons, etc when pushed out to about 90deg). If this is the case, your closer is probably worn out and you need a service man.
Removing and replacing a glass entrance door to access a closer is not an amateur project. These doors are heavy and can kill someone if they drop out of the frame onto someone while you are working or years afterwards. You can adjust it if you can find the screws, but most any other service is worth the investment in a glass entrance contractor. Dont let the door slam as you are adjusting it's speed, etc , etc All standard warnings apply.
