Go to My Documents\My Games\Borderlands\WillowGame\Config\ and open WillowGame.ini. Search for the term and change it.

Cap Weapon Slots = WeaponReadyMax (2 to 4)
Demigod Mode (Invincibility) = bDemiGodMode (set to true)
Raise Max Inventory = InventorySlotMax_Misc (choose number)
set to "false" to be able to shoot your teammates and do damage (there's two of these) = bIgnoreFriendlyFire
set to "false" to let NPCs do dmg to eachother = bIgnoreNPCFriendlyFire
set to "true" so bosses do not regain health after dying. = bBossesRegenHealthOnReset
set to "true" to ressurect all players when one of your guys dies. = bResurrectAllPlayersWhenOneDies

WillowTree is a software utility used to modify Borderlands save games.

https://borderlands.fandom.com/wiki/Willowtree