How to fix a seized V-Brake arm?
V-brakes aren't too complicated but there are a few things to watch out for:


you need quite a lot of torque to undo the bolt 
the spring may make it fall apart sooner than you expect - hold on as you undo 
make sure all the parts go back in the right order - take care as you dismantle.  Work with a tray underneath to catch anything that falls. 


Once you've taken it to bits, clean all the muck and loose rust off the parts, the grease and reassemble. The bolt should be tight. You'll probably need to do the other arm as well, and may need to adjust the return spring screws. Obviously don't get grease or greasy hands on the pads.