modern sportbikes have a keylock on the seat, simply unlock the seat and slide it out of its brackets. Other seats are attached by several bolts under the the seat.
This in usually not a universal process. Every bike may have the seat attached in a different way. Most newer bike have a set of bolts holding the seat on. Some have seats that pivot up to allow access to the hardware. Check online for specifics about your bike.
Changing seats on a motorcycle requires knowing what other aftermarket seats are available for your particular model motorcycle. Not any seat off the shelf is going to fit. If you're not able to get the information from your local dealer, you can find it on the Internet. Using another seat or making your own will require modification.