As
@Stanley Orchard said your CTR is sacrosanct pretty good but to improve you just have to make your content more clickable. Ensure you know who you're audience is and give them exactly what they want. This you can know based on who's watching and what exactly about your content they enjoy the most. I've got can figure this out you can then give them more of that. The same with the thumbnails, as Stanley also said.
Basically, there is no special trick or specific thing to do that can change your CTR higher or lower. It's all about the content and getting your audience to click through now of your content. Make out clickable and watchable.