Once two sections of PVC pipe are joined using a glued PVC fitting they are welded together. There is no solvent or "de-gluing" agent which can be applied to soften the glue and allow the pipes to be pulled back apart. In a situation where a pair of PVC pipes need to be separated, a more drastic approach will be required.
Use a PVC pipe cutter to cut one pipe or the other as close to the glued fitting as possible. If the pipe is in a position where the PVC pipe cutter can't be used because of insufficient space, a hacksaw will easily cut the pipe.
Cut off the other pipe as near to the glued fitting as possible if the goal is to eliminate the fitting entirely or perhaps replace it with a different type of fitting.
Cut the pipes apart at the fitting if necessary. If the fitting is a straight coupler, a PVC cutter can be used. If it's a tee or elbow fitting, use a hacksaw. A part of the fitting will remain on each end of the cut-apart pipes.
Remove the remnant of the glued fitting by grinding it away with a coarse wheel on an angle grinder. If this end of the pipe is to be reused with another fitting, be as careful as possible to remove only the plastic from the fitting and none from the pipe.
If cutting away the entire fitting leaves the remaining pipe too short to be reused, it's a simple matter to move back a few inches, cut the pipe again then use a glued coupler to attach new piece of pipe to the old one with the new section cut a bit longer than the end which was removed.
Removing a fitting and reusing the end of a pipe to which it was attached is not likely to produce a watertight connection. Use this method only if the PCV pipe is being used for ventilation or in a non-plumbing application such as PVC lawn furniture or as a framework for an outdoor banner.