If you’re getting water inside, Jchannel is probably the least of your concerns. Something isn’t sealed correctly. Unfortunately, if you have water getting in, you have water in your wall. Even if you seal the window, you could end up with a mold issue.

If it was me, I’d pull the window, ensure all flashing and housewrap is in place, the reseat the window in a thick bead of silicone. Assuming the window is still in good shape. Otherwise, a new window is needed. Once the window is in place, trim, jchannel, and siding would be put back.

Once the window is properly installed, I’d focus on the inside. Pull the trim and drywall around the window, letting it air out for a day then coat the inside sheeting with mold killing primer. New insulation would be added before new drywall is put in. Then mud, sand, paint, and reinstall the trim.

It’s a decent amount of work, but not insurmountable. My concern is, if it was installed incorrectly, chances are, your other windows were too. I’m not trying to be doom and gloom here, just trying to forewarn you. It’s something to keep an eye on.