Breakwellmz is right in that I cannot judge properly what to do with this issue without seeing it properly. Similarly I cannot judge if your sprockets & chain are worn. I see a ridge of old lubricant & grime on the sprocket, but the teeth might not be all that bad - I can't see. By comparison, these are worn sprockets:
An 'O'-ring chain usually doesn't stretch all that much, and normally what goes first are the rollers on the inside, which follow the sprockets.
If those show much wear (=clearance) on the bush inside them, as you should feel with your fingers to test, that's a reason to replace it. On a young chain, those rollers have little or no clearance on the bush inside.
But I cannot even see if yours is an 'O'-ring chain. A normal chain stretches much quicker, a common guideline for replacement is 1% stretch for an 'O'-ring chain, and 2% for a normal non 'O'-ring chain.
Another common check is to try to lift the chain from the sprocket, in the middle at the back, while there is no tension in the chain. If you can lift it significantly, they're worn. In a good set you can only pull it up a few mm.
