Welcome to Android Central! You mean Sprint, right? First, you have to make sure it's unlocked--see the Sprint policy:
Legal / Regulatory & Consumer Resources
However, the problem is that the Sprint M8 doesn't support the 4G LTE bands that Telcel uses (which are 1700 MHz and 2100 MHz). The Sprint M8 can use 850 Mhz, 1900 Mhz, and 2600 MHz. The phone could probably get 3G, since Telcel uses 850 MHz for that (which the phone supports).