To safely change a circuit from 15 amps to 20 amps you'll need to replace all of the wiring to 12 AWG and replace all outlets that may be underrated. DO NOT simply replace the fuse with a larger one....
Yes, you can change a fuse in your electrical panel with out the help of an electrician. Do a little detective work first to establish what took the fuse out. If something shorted out, remove it from...
The fuse diagram should be in your owners manual, and also on the fuse box cover. If you don't have the manual, and there is no diagram on the fuse box cover, go to your local Nissan dealer and ask...