Level up your Pins by collecting duplicate Pins from Gear Packs. Once you have the required number of Pins, you can upgrade the Pin with Coins. The higher the level, the more Pins you will need to collect and the more Coins it will cost.
Example: to increase an Ink Grenade from level 2 to level 3, you'll need to collect 15 Ink Grenade Pins first before you can use 120 coins to upgrade. A Level 3 Ink Grenade will require 23 more Pins before it is eligible for upgrade again.