UART 1 is on pin 8 and pin 10 with optional flow control on pin 16 and pin 18.
sudo ldto enable uart1
sudo stty -F /dev/S1 921600
UART 2 is on pin 36 and pin 38 without flow control pins. UART 2 is temporarily enabled by the bootloader for debugging so it should not be used for sensitive peripherals or those without reset as the output data from the bootloader may put the peripheral in a locked state. We recommend using UART 1 for peripherals.
sudo ldto enable uart2
sudo stty -F /dev/S2 921600
To permanently enable UARTs on startup, replace enable with merge.