remove the front axle and wheel
leave the tubes in the tree's as this will help you get the top cap off
the lower legs of the forks have an access at the bottom to an socket head bolt (allen head)
remove the socket cap and let the oil drain fully then reinstall bolt and tighten
do both sides and then reinstall wheel and axle.
Undo the top fork cap and pour in the correct amount of oil.
Usually people will tear the forks down to clean out the oil and residue and install new seals as well, but if your seals are ok then just drain and refill