Upvote:5
It does depend on what kind of visa you were issued, but if you are a tourist on the standard FMM (Forma Migratoria MΓΊltiple), and you were given less than the standard 180 days, and you need longer, you can go to an immigration office (INM) and ask to have it extended. You will need all your documents - current visa, passport, reason for extension (and proof that you only need a small extension), and especially proof of ability to support yourself for those extra 9 days.
I have not had to do this, but I have heard that this is a pretty painless operation (as much as it ever is with Mexican bureaucracy). It's not like the US or UK where they're immediately going to assume nefariousness.
Another option is to leave Mexico and come back in and get a new time on a new FMM. But that requires being able to enter another country, and can be seen as suspicious (why are you wanting to spend more time here than we think is right? Better to show the committed flight home to the immigration officers and explain why.)
As commented, it is best if you clear this up as soon as you can, rather than letting the visa expire. Not only are there fines, but you may not be allowed back into Mexico; and you may find it more difficult to get a visa to the more suspicious countries, even years later. Much better, is to get the visa extended. Next best, rebook the flight.
Note that (barring emergencies) it is not possible to extend a tourist visa past 180 days.
A lot of this information is mentioned at this commercial site, I have not yet found the official government information.