How to extend your tourist card:
Visit the nearest Mexican immigration office. Here is a list: Offices of the Instituto Nacional de Migracion.
You will be asked to show your passport and valid tourist card, as well as proof that you have sufficient funds to support yourself during your stay in Mexico (credit card or bank card, travelers checks and/or cash).
You will need to fill in the form you are given in the immigration office and take it to the bank to make a payment, and then return the forms to the immigration office.
Make sure to get there early enough to complete the whole process (including possibly long line-ups at the bank and immigration offices). Immigration office hours are Monday to Friday 9 am to 1 pm, closed on national holidays.
More about tourist cards:
What is a tourist card and how do I get one?
What do I do if I've lost my Mexico tourist card?
