If the spark plug has fouled you can sometimes use a wire brush on a bench grinder to clean the spark plugs back up and remove any carbon deposits from the spark plugs.
However cleaning the fouled spark plugs is usually not enough to make the vehicle run properly again and I would recommend just buying a new set of spark plugs which can sometimes be bought $25.00 or less for the set of spark plugs.
I've cleaned fouled spark plugs before to get by with my vehicle until the next payday but when I got my next pay I replaced the fouled spark plugs with new ones and the vehicle ran much better.
So if you need too you can sometimes clean the spark plugs enough with a wire brush and the vehicle should run a little better but still you should replace them when you can.