Of course you can.
remove the seat
There are a number of places where one can purchase a Harley Davidson battery. For instance, JP Cycles, Throttle Batteries, and Battery Mart all carry Harley Davidson batteries.
One can purchase a replacement battery for their Harley Davidson motorcycle at their local Harley Davidson store, online or a private shop that specializes in replacement batteries.
Under seat.
There is a special spot where you are suppose to put the battery and if you really don't know where the spot is, look it up on You Tube.
under the Seat
No, they are covered by the battery manufacture.
12 volt.
12 volts
Low Battery
Remove seat, un bolt the battery, pull up and remove. Put a new one in, bolt the two knots and replace the seat!
You can take it to a battery place and they can test it under a load.