If it's not visible from above then you must remove the trap j-bend. Go under the sink and put a big bowl or tray under the trap because when you remove the j-bend water will come out. You'll need a 12" grooved pliers, like channel locks, to get the nuts off. There are two of them. If the trap is old brass or chrome you may end up cracking the j-bend. If it's white or black plastic you can probably remove the nuts by hand. Look in the j and pull out your stopper. If it's visible from above a long pair of needle nose pliers would work best to remove the stopper.
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If it's not visible from above then you must remove the trap j-bend. Go under the sink and put a big bowl or tray under the trap because when you remove the j-bend water will come out. You'll need a 12" grooved pliers, like channel locks, to get the nuts off. There are two of them. If the trap is old brass or chrome you may end up cracking the j-bend. If it's white or black plastic you can probably remove the nuts by hand. Look in the j and pull out your stopper. If it's visible from above a long pair of needle nose pliers would work best to remove the stopper.