Indeed, it uses 866.550 SF10BW125.
When using the public network, you cannot change that. However, if all is well, then TTN should use the LoRaWAN defaults for the Join Accept and then, in that Join Accept, include any network settings for anything that is not standard. But I think the TTN frequency plan is just always using the LoRaWAN defaults:
The RX2 receive window uses a fixed frequency and data rate. The default parameters are 866.550 MHz / DR2 (SF10, 125 kHz).
So, why do you think you need changes?
If you’re asking how to enforce RX1: it’s up to the TTN network server to choose either RX1 or RX2, so you cannot really enforce either. But if you use a low SF for the Join Request then chances are better that TTN will use RX1 for the Join Accept, as TTN prefers RX1 as well, at least for EU868. Still, a LoRaWAN compatible device should work with both RX1 and RX2.