How do you replace a Remington 870 forend?

Answer 1

assuming you want to replace just grip of forend

Take barrel off by unscrewing cap at front of magazine and pulling barrel off

take out mag spring and follower from tube magazine

without barrel or magazine contents hold shotgun receiver upside down in lap with feed port facing up

inside the receiver is a metal rail which you have to push in before you can slide off forend, reach into receiver from loading port and rail should be on right hand side when looking at receiver pointing away from you and upside down.

when properly depressed forend should click and slide off away from receiver bringing with it the bolt and bolt carrier

to remove grip from guide rails unscrew nut with 2 notches in it at end opposite to rails and slide grip out

terminology may be different in manual but you should be able to follow hope this helps