The S805 relies on a proprietary kernel tree. A direct upgrade to a modern mainline kernel (4.x or 5.x) is impossible due to the closed-source nature of the Mali-450 drivers and VPU (Video Processing Unit) codecs.
Installing Android 7.0 on Amlogic S805 is relatively straightforward. Here's a step-by-step guide: amlogic s805 android 7 hot
After scraping forums (XDA, FreakTab, Reddit r/AndroidTVBoxes), here are the two most successful solutions for overheating S805 boxes running Android 7: The S805 relies on a proprietary kernel tree