For information on getting your Mexican resident visa, changes in your visa (if you get a job, get married, move, etc.), and filing paperwork for the visa, visit the National Institute of Migration.
In Saltillo, they’re located in Colonia República, two blocks east of Venustiano Carranza on Calle Colima.
Instituto Nacional de Migración Colima #213 Col. República Saltillo, Coahuila 25280
Or, visit the INM website at: https://www.gob.mx/tramites/migracion-visa-y-pasaporte
It may also be possible to apply for a resident visa before entering Mexico. Check with your nearest Mexican consulate. However, this may not be possible in all cases.