You can get a player's inventory using the EntityPlayer#inventory field. You can use InventoryPlayer#setInventorySlotContents to replace the ItemStack in a slot of the player's inventory. The armour slots are 36-39.
To do this from a keybinding, you may need to send a packet to the server or use a Container. I'm not that familiar with the networking system, so I can't help you there.
Please don't PM me to ask for help or to join your mod development team. Asking your question in a public thread preserves it for people who are having the same problem in the future. I'm not interested in developing mods with people.