Use the nrfutil 93-series command to perform modem firmware upgrade with the nRF93M1 module using AT commands over UART.
This tool requires that the modem's AT command interface is exposed on a serial port on your device. For example, with the nRF93M1 DK, this can be achieved by running the Modem bypass sample from the nRF Connect SDK on the nRF54L15 host chip. Use the nrfutil device list output of the nRF93M1 DK to figure out which serial port to use.