Near the opposite end of the dashcam from where you removed the plastic window, you'll see a small opening on the side of the end-cap.

Using a small flat bladed screwdriver, press on the plastic tab that you find inside that opening.

Pressing on that tab should release the plastic end-cap so that you can slide it off the end of the dashcam's cylindrical body.

The 3rd photo shows the end-cap after it's been removed.