There's no quick and easy way to do this. Remove the screws and loosen the J rail. If you find rot, it will need to be repaired. The front piece should fold over the siding. Re-putty the J rail and re-attach.
FYI: all the caulk in the world will not prevent leaking.
I don't think the bottom piece is original. Shasta used either a criss cross small pattern or something that matched the siding. I think a piece of aluminum would be much easier to install and the edges easier to bend.
The cracks running up from the tongue suggest damage to the framing behind. You might run into more repairs than expected. Have fun with it.
Not sure how you are doing with your trailer. Hope all is going well. This is a late post but after reading and looking at the pics, it looks as if your trailer is sagging in the walls causing the metal to bend on the piece shown by the tongue area around the frame. Check closely on your side wall. There may be enough rot there to cause the wall to drop down. Best of luck.