DIAGNOSIS The shoe (that metal plate you rest against the work) is held by a single large bolt and a pivot. The bolt has likely loosened up from vibration, or the locking lever mechanism is worn.
FIX Flip the saw over. You'll see a large, flat-headed bolt in the center of the shoe. You need a hex key (Allen wrench) that fits it—it's usually a decent size, like 5mm or 6mm. Hold the shoe in the position you want it, then tighten that center bolt firmly. Don't crank it with all your might, but get it good and snug. That should take the slop out. If it has a separate locking lever on the side, make sure that's also tightened down.
IF THAT DIDN'T WORK If tightening the bolt doesn't stop the wobble, the pivot post or the shoe itself might be worn or damaged. The metal-on-metal contact can wear a groove over time. You'd need to replace the shoe assembly, which is usually a single part that includes the pivot and the bolt.
HEADS UP When you adjust the shoe for a fresh section of blade, always re-tighten that bolt. It will vibrate loose again if you don't. A little drop of blue (medium-strength) threadlocker on the bolt threads can help keep it in place without making it permanent.