Dewalt circular saw, motor works but blade doesn’t turn

It is very likely, that the fixating screw/disk is loose. Do not operate the saw in this state. It is dangerous.

A circular saw usually has a direct drive. I.e. the motor spindle is also the center shaft for the blade. So if your motor is spinning, but not the blade, it means the blade has lost contact to the shaft.

Check if the blade is properly centered and aligned with all locating pins (if your saw has some). Then fixate the screw using the appropriate wrenches.

In general, it is not worth taking apart tools such as a handheld circular saw or drill. There is not much in the way of user-serviceable parts, and doing so may void any warranty you have with the manufacturer.

First, I recommend looking up if there are any "consumable" parts such as fuses which are user-serviceable. This may be a quick and easy fix.

Your next step should be finding an authorized DeWalt service center that has the specialized tools, parts, and training to repair the tool. The major tool companies tend to have company-owned or authorized third-party repair centers in major metropolitan centers in the U.S. at the very least. They can inspect the tool and tell you if repair makes sense: it may cost more to repair the tool than to replace it. Being this new, however, it is likely covered by a factory warranty that will cover a significant portion of the cost.