Whether or not a blister11 needs any treatment is debatable. If the blister is small, unbroken and not very painful, it is probably best to leave it alone. Cover it to prevent continued rubbing and pressure on it that can cause it to swell more and possibly burst on its own.
If the blister is large or painful—especially if the activity isn’t finished (such as you are in the middle of a hike)—follow steps to drain and dress a blister.12 Use a sterilized needle and make small punctures at the edge of the blister and express the fluid. Then apply antibiotic ointment and cover it to protect it from further rubbing and pressure.