Aluminum gutter end caps are a little different to repair than the standard assembled gutter runners. These types of gutters simply have a cap or a plate covering the end of the gutter as a stop to prevent water from rushing out.
Step 1 - Take the Old Caps off
Take your rubber mallet and gently tap the gutter end caps off the gutter itself. Hit on the edges where it attaches to knock it loose. You can also use your screwdriver on the outside edge to apply a little leverage if needed to get them loose.
Step 2 - Clean off the Old Caulking
Around the area where the gutter end caps were attached, take your flat screwdriver and peel the old caulking or sealant from its edges.
Step 3 - Apply Sealant to the New Cap
Once the area where the gutter end caps were has been cleaned up, take your new caps and put an even bead of silicone into the grooved channel around the edges. Once the slotted area of the cap is properly holding sealant, push the cap over the end of your gutter until it sits evenly. Wipe away any excess sealant, and let it dry.