You could go into you video analytics on the creator studio app, "go to video analytics" to to the discovery tag, and then look at your impressions clickthrough rate. if it's low, then you can work on improving your thumbnail (I have a really high performing video with about 20k impressions in the last 28 days, the click through is 3.2 so I guess that tells me I could improve my thumbnail or maybe improve my title to make it more clickable)
you could also check how your tag scores are in the tubebuddy app and if there's room for improvement in your tags, you could improve them a bit more.
Just remember though, "improving" videos comes with a risk. You might THINK you have a great new thumbnail that might perform better, but it may end up performing WORSE (I've seen this happen) so generally, if your video is performing really well... make more of them.