According to the Mexican Law all your immigration papers must be in force and the information in them contained must be updated during your stay in the country. In case they expire or the information is not provided to the authorities within 30 days of the event that modifies such information, you will be liable for fines and other penalties. It is highly recommended that you keep this in mind. You may try doing it yourself. Yet, if you are not familiar with the paper work, the Mexican governmental offices, the bureaucracy and especially the language, it would be wise to ask for help.
Every procedure generates a cost.
Rates change every six months and may be paid for at any bank in Mexico. You will have to fill in certain official forms to make the corresponding payments. These may be requested at any migration office or you can even buy them at a stationary store. They have years of experience in their respective professions, as well as years of experience in dealing with expatriates. They are fully bilingual and guarantee some of the best service in town.