It's difficult to advise without seeing it. Most belts are of the webbing variety that feed through a plastic coupling buckle type thingy. It's easy to pull it and make it longer, but shortening usually requires feeding the webbing laboriously through the bar and pulling it up on the reverse side, bit by bit, if you see what I mean?
What make is it? You should be able to get the instructions on line. I did and downloaded the fitting instructions onto my tablet.
With the one I have, (a Britax 123plus for over 9 month olds.) There is a lever under the child seat where the bottom part of the buckle is.
If you push that lever down with one hand and pull on childs seat belt straps themselves (not the padded covers) you should be able to loosen them.
I have spent the last few week practising to get my DGS's in and out of the car quickly. The first time I tried getting it in and out of the car I made a dreadful mess of trying to get the seat belt threaded the right way through the carcass of the seat. Looking at the way DDs works most of them seem the same.