put your thumb over the white tip and hit the release button... The plunger should unlock and attempt to drive up, and you should feel suction at your thumb. If no suction, then the solder sucker is either dead or really dirty.

Often the plunger will unlock from these with a quarter or half turn, and you can remove the plunger and core. There is an o ring that goes around a stacked disk often gets coated with old solder dust, which reduces the ability of the sucker to pull a vacuum. Clean off the old solder, lightly coat the o ring with silicon grease and put the thing back together and it should work fine.

Had one form or another of these in my tool kit for the past 30 years.

These days I use more solder braid wicking than a solder sucker, but those suckers still are handy from time to time.