Instructions
Press "C" to open your character sheet and "I" to open your inventory.
Click and hold the left mouse button on a relic in your inventory, located on the bottom right corner of the screen.
Drag the relic to your ranged weapon slot, located at the bottom of your character sheet on the left side of the screen. The ranged weapon slot is located next to the melee weapon slot.
Release the left mouse button to place the relic in the slot. The relic bonuses are automatically applied to your character as soon as the relic is equipped.