You will be able to see the Perks you have available in your Hero select menu before starting a level.
To equip a Tower perk simply drag the Perk you would like to the vacant tower slot you should be able to see on your right-hand side, to swap this Perk simply drag another one into the same slot to replace it. If you change your mind about using a perk, you can also tap any equipped Perk to remove it.
To equip a Hero Perk, drag the Perk onto the already selected hero you would like to use for the level! Be aware that each Hero Perk is specific to that hero with even different hero skins having their own specific Perks!
Just remember, Perks are consumable which means you can only use them once so choose use them wisely!