If the fare allows changes, you can ask for exchanging your ticket at the station counter:

The options are listed below:

AVE and Larga Distancia (long distance high-speed) / Avant (mid-distance high-speed) / Media Distancia (mid-distance) trains:

• Premium: Free change included to be made no later than 30 minutes after departure.
• Elige Estandar/Confort: These tickets can be changed no later than 30 minutes after departure by paying the 30% of the new ticket.
• Basico (Basic Fare): This is the cheapest fare which unfortunately cannot be changed.

AVLO (high-speed) trains:

• Basico (Basic Fare): Changes are not allowed.

For more information, please refer to related article How can I change my ticket?