Changing weapons in a map?

Well, that's simple. First off, you need to know actually what to search for. You can either use the actor search (more safe) or navigate there in the 3D window (easier). So if you want to replace all weapons of some class, and you know their names (open the actor browser and look for them if you don't), you can use the Actor Search function (looking glass @ top bar). Do a search and it will give you all options. When clicked, the editor will transport your camera to the actor. Then in the 3D viewport (lower left) right-click the actor and select Delete.
Now if you want to do only specific changes, you need to use the 3D browser (lower left corner). It's controls are kinda tricky though. You move around by left clicking on a viewport and then dragging your mouse to one side. Try playing with that. You look around by right clicking on the viewport. You go up and down by left and right clicking together on the viewport. Now when you are used to flying around, go to the place you know the actor is and delete it. If you want to add an actor, you need to find it on the Actor browser (pickups are in Actor -> Inventory -> Weapon and Actor -> Inventory -> Pickup -> Ammo). For example, the Redeemer is at Actor -> Inventory -> Weapon -> WarHeadLauncher (probably a codename ). Select it, right-click where you want it to be and click "add *NAME* here". After that, save the map with another title (otherwise you'll get nasty version mismatches). You could also rebuild the map after that (cube with a light bulb at the top bar) for best bot behaviour. Then save again.