Have it done by the boat yard.  Give them the rub rail and have them do the work.  

I'm assuming you just want to replace the rubber piece. I replaced mine myself while boat was in the water.  Not that hard a job.  But your lying on the deck and straddling the stansions. Easier with two people. Mine snapped in with just pushing hard with your fingers.  Getting about 2-3 inches in either the top or bottom groove, then pressing in the other side.  You could also use a rubber mallet, but found that more cumbersome to swing while lying on your stomach. Also suggest unrolling the whole new rub rail and let it soften in the sun. This really helped when wrapping it tightly around the bow. Keep pulling it tight as you go to minimize stretching later on.  I found that after a few rubs it stretched and I needed to redo a section, which ment reinstalling it all the way to the aft end. I also tried a soapy solution but don't know if it helped much. That made if slippery to work with. Cuts easily with a razor knife. One screw on the end cap to replace on each aft end to finish the job. Mark the screw hole on the metal rail since it is hard to find later.  . 

I tend to agree with Lou here. I hit a dock once and the rub rail bulged out of the track. I sliced it and fit it back in, cutting off a bit so the ends matched. After a short while, the darn thing retracted and I had a gap between the two ends. I could see something similar happening  where, if you trimmed off the end after getting the rubber into the track, it shrinks up and it becomes too short. 

If you do do it yourself, I'd suggest after getting the rubber part into the track, let it extend out the back end for a week or so (I;m guessing on the time, maybe a few days will work) before trimming it and putting on the end cap.
 

I used a white rubber mallet, worked great. Avoid a black rubber mallet as it will leave black marks.