Change Arduino Clock Speed
You have to change F_CPU only - You can add new board into the boards.txt with correct clock speed.
EDIT: Aparently millis counter will be drifting much more then expected due to "rounding" errors.

The bootloader on the other side needs to be recompiled with correct F_CPU and  reflashed, if you want it working at the same baud rate (plus it's possible to change fuses to use external clock source instead of crystal). Otherwise you have to try reduced speed (also configured in board config).

You can use MCUdude/MiniCore as inspiration. It supports many clock speeds (but 10MHz doesn't)