The term "reboot" simply means to restart a device.
What you're asking for is "how do I do a factory reset of my router?".
In that case, most home routers have a "reset" button on the back that you have to push using something small like the tip of a ball-point pen.
Use the ball-point pen tip to press the reset button down for 10 seconds with the router still plugged-in to the power supply. While still holding the reset button down, unplug the router's power supply and wait 10 seconds. After that 10 seconds is up and while still holding the reset button down, plug-in the power supply to the router and wait yet another 10 seconds and then release the button.
This will return your router to factory defaults. Unfortunately, you didn't tell us the make or model number of the router, so I can't tell you for certain the URL to use to set up your router again.
Most likely it is either http://192.168.0.1/
with some default password.