How Can a User Replace a Zero-Fee Transaction with a Higher-Fee One?

A user can attempt to replace an unconfirmed transaction using a method called Replace-by-Fee (RBF). RBF allows a user to broadcast a new version of the transaction with the same inputs but a higher fee.

The original transaction must have signaled RBF eligibility, or the network must allow "opt-in RBF." The miner, seeing two conflicting transactions for the same input, will choose the one with the higher fee, thus replacing the original zero-fee transaction. This is a common way to "unstick" a transaction that was initially broadcast with a low or zero fee.