To remove the stock narrow forearm you simply tap out the retaining pin that attaches it near the triple tree, wiggle the handguard a bit, pulling it down and away as it is also retained on the front of the receiver by "Friction and a snug fit" only. Once you've removed it place the Wide forearm onto the receiver in reverse order. You should have received an instruction manual that shows all this by the way, last things guys do BTW is read the instruction manuals. LOL
BTW now that you have it broken in give the 115's a try as well. They work just as well.