How to change the baud rate of the Serial Monitor in Arduino?

In order to change the baud rate of the Serial Monitor, go to Tools -> Serial Monitor (or press Ctrl+Shift+M alternatively).

Make sure you have a board connected to your PC/Laptop, or the Serial Monitor won't open. Also, make sure that the Port corresponds to the connected board.

Once the Serial Monitor is open, you can see the baud rate dropdown at the bottom and select the required baud rate from there.