Your phone will automatically switch to TELCEL or Movistar, depending on who has the greater signal in the area you will be.
To manually change the service provider: Go to settings, connections, Data and turn off Mobile Data (having this on wont be able to switch successfully).
The search for "Mobile Networks", then "Network Operators", select "Search now". It will take around 1-5 minutes and will display all available options.
Select the Mexican carrier you desire to use.
Note: If you select Telecel but has fewer signal than Movistar, your phone will automatically swith to Movistar. If you want to avoid this, before selecting "search now" there is a "Default Setup" option. Select it and it will ask "automatic or manual". Select manual and then do the search now. The manual is indicating you will manually select the carrier and for the phone to do not search automatically and switch. You will need to select "automatic" back when you are at the US