Since a coin flip is a binary outcome, you can make an unfair coin fair by flipping it twice. If you flip it twice, there are two outcomes that you can bet on: heads followed by tails or tails followed by heads.
P(heads) * P(tails) = P(tails) * P(heads)
This makes sense since each coin toss is an independent event. This means that if you get heads ? heads or tails ? tails, you would need to reflip the coin.