Tighten the pivot screw in small increments with the correctly sized Torx driver, testing after each turn. Aim for the loosest setting that shows no side-to-side blade movement when locked open. Most pivots are T6, T8 or T10, and using a bit that is slightly too small is the fastest way to strip one.
Side-to-side play at the blade usually means a loose pivot. Play back and forth while the knife is locked open, sometimes called lock rock, is a different problem and tightening the pivot will not fix it. See lock play.
Loose handle screws or a loose clip can also feel like a loose knife. Check those before touching the pivot.
Use the largest Torx bit that seats fully. Many pivots have factory thread locker, so the first turn takes real force; that is normal. Turn a small amount, close and open the knife, check for play, repeat.
Overtightening makes the action drag and can stop a flipper firing entirely.
A single small drop of removable blue thread locker on the threads, allowed to cure, solves it. Never use permanent red.
Note that some makers treat disassembly as outside the warranty. Adjusting a pivot is normally fine; taking the knife apart may not be. Check before you strip it.