You may need to remove your mountain bike’s pedals at some point, whether it’s for maintenance or to replace them.
Most pedals need a 6mm or 8mm allen key, although some require a pedal spanner. Once you’ve located the right tool, try loosening the pedal. Be careful as, if the pedal wasn’t greased before fitting, it may be very tight.
The key thing to remember is both pedals have threads that rotate in different directions. The one on the drive side (with the gears on) has a traditional thread and you need to rotate left to loosen. For the left-sided pedal you need to rotate right.
Make sure you rotate the correct direction otherwise you could end up overtightening the pedal.
You can find all the tools you need including allen keys and pedal spanners here