You can do this by setting the data rate: bool dataRate(uint8_t dr)
dr can be between 0-6 (I have tested this, and 6 doesn't work, but all the rest does)
/*
DataRate Modulation SF BW bit/s
0 LoRa 12 125 250
1 LoRa 11 125 440
2 LoRa 10 125 980
3 LoRa 9 125 1'760
4 LoRa 8 125 3'125
5 LoRa 7 125 5'470
6 LoRa 7 250 11'000
*/