Here's a quote of the relevant section of David Chicoine's book, Gunsmithing Guns of the Old West:
(After removing the buttstock),
"Open the lever only far enough to give you access to the mainspring screw which is the rear-most screw on the lower action tang, loosen and remove the mainspring screw and the mainspring may be lifted out of its seat with the hammer stirrup and out of the frame."
There are no details given for reassembly of that part. Maybe that will help.