Join Date: Jan 2019
Location: Central North Carolina
Most bunk beds that I have seen are designed with the bed posts pinned together. In other words, made separately with holes in the top and bottom of each post. Then there are steel pins that get inserted into the top end holes and the bed posts for the upper bunk fit down on these pine. Even 3 bunks high is possible with this design.
Unfortunately, I see no joint in your posts, so the bed will likely need to be dis-assembled and then modified by sawing the bed posts off at the desired height, or making short replacement bed posts of the desired height. The round buttons are covers for recessed screws. You will need to pop them off to get access to the screws. Once the screws are removed, you should be able to dis-assemble the bed.
Now the woodworking begins. If you cut the existing bed posts, you are going to permanently loose the upper bunk. If you make short bed posts for just the lower bunk, copying the design as closely as possible, and storing the upper bunk, long posts, and ladder, you can some day convert the single bed back into a bunk bed. The design looks simple enough that you should be able to make short bed posts quite easily. Some higher quality construction grade lumber, a saw, drill and bits, and sandpaper are all that would seem to be required to build them, plus a finish to match the original.