How do I update this Theme?

So I purchase your theme licencse over and over again every year. However I can never simply just update the theme. Im afraid to Download the full theme and Re-insstall it becuase I dont want my website to disapear. Why cant we just simply update the theme. The only way is for me to download thew whole thing and reinstall everytime? My wordpress says there is a update available, but no simple way to click a update button etc. If there is a new version with better tech id like to have it. But I dont want to completely gut my site everytime. please tell me you have a solution for this?