so im getting new hiking straps i was wondering if it is easy to replace it, or do you i need silicon or something, please help show a video or something if possible thank you.
Essentially yes. And of course it's good to use silicone (or some other sealant) in every hole that goes through the skin of a buoyancy tank (which in the case of the Laser is the whole hull)! The thing is that you don't need very much of it; you're not trying to glue the strap in place. Scrape off all the old sealant first and apply just enough of the new to fill the holes.
You may also want to open up the fabric of the strap with a spike at the points where the screws will go through, unless it already has holes there.
4200 works great, the strap is quite easy to replace. Just take out the screws but try to leave the whole unit of the mainsheet block, spring, fairlead and plastic plate in one piece.