The flippant answer is to dismantle the charger - find and test the fuse - and to fit a new fuse...
However - the fuse will have blown because of a reason - perhaps a dead short on a battery being charged - perhaps because of a defective component inside the charger - it's rare for a fuse to fail for no reason at all.
So if you are able to fit a new fuse there's no certainty that the new fuse will not also blow.
If it were me I would take it apart and check what components that I could check but I would have no confidence that I would make the charger work again.
Good Luck...
June 2017
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