You need to be more specific and identify a manufacturer and model, i.e. Winchester Model 94, Marlin Model 39, etc. For the most part, you don't want to dissassemble beyond what the owner's manual recommends for cleaning.
marauder.homestead.com
Best left to a trained gunsmith
You really should leave this to a trained gunsmith.
You can download a manual here - http://www.henryrepeating.com/freestuff.cfm
To disassemble a Browning Model 1886 lever action rifle for cleaning, first ensure the rifle is unloaded. Then, remove the barrel by loosening the magazine tube band and sliding the magazine tube forward. Next, remove the lever by pushing out the lever pivot pin and then taking out the bolt and carrier. Finally, clean all components thoroughly before reassembling the rifle in the reverse order.
how can i disassemble this gun
You need to find a gunsmith.
If you are not trained to do this, I recommend you do not try it. Chances are better than even you will be taking a box of parts in to a gunsmith and that's not good.
Since there several different makes and models, and they are all different, contact the maker thru their website, and ask for the owner's manual that will show you how.
Marlin 336 Owners Manual at Marlin Firearms website: http://www.marlinfirearms.com/pdfs/manuals/MFC_Centerfire.pdf
Revolvers are not lever action firearms. Sorry- can you rephrase your question? Lever action firearms are most often rifles, rarely shotguns. A very few lever action pistols were made, but not revolvers.
To disassemble a 20 gauge Beretta 390 shotgun, first ensure the firearm is unloaded. Remove the forend by pressing the forend release lever and sliding the forend forward. Next, open the action and remove the bolt by pulling the bolt release lever and sliding it out of the receiver. Finally, to separate the barrel from the receiver, locate the barrel latch, press it, and pull the barrel away from the receiver.