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
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 Darne shotgun, first ensure the firearm is unloaded and safe. Remove the fore-end by pressing the release lever and sliding it off the action. Next, open the action to expose the barrels, then unscrew the barrel hinge pin to separate the barrels from the receiver. Finally, take out the trigger assembly and other internal components as needed, following the manufacturer's instructions for any specific details.
Try the related link >>>> It doesn't explain HOW it works, but gives the steps to completely disassemble one.