The inner seam is a little hard to get to because it is behind a rubber wipe seal. It is hard to describe, but you can hold it out of the way as you work along the seam. Another approach is from the inside. I watched a Beaver tech put a piece of standard plastic tubing on the calk tube as an extension. He fed the tube over the top of the slide molding and sealed it by feel.
Besides the inner and outer seems, mine has some screw or rivet holes with small dabs of calk. Since the top of the slide is flat, there are always places the water pools. If one of those small holes is not water tight and water is pooling there, the slide leaks. Make sure the rivet/screw holes are well sealed.
Another approach beside removing the topper is to go at it from the ends. I've not tried it but I watched a tech unreel some of the extra topper and hold it with clamps so it would not reel back up. Then he slid under the loose fabric on his stomach! It worked for him!
My approach was to extend the slide and clamp wood blocks to the fabric to prevent roll up. Then I removed the brackets on the side of the slide that are held with three screws each. Next I folded the whole assembly up on the roof. Depending on the slide it can help to use rope to hold it there while you work. The ropes can attach to anything from the ladder to the tank vents. It would be easier with two people, but I substituted the rope for the helper!
Good luck. Don't fall off the ladder and be sure the topper spring does not retract with your fingers in the way!