Q: How to downgrade ios 10 beta to ios 9.3.4 ios 10 beta make my iphone 6 very lag

Always back up your iOS device before installing beta software. To get a shipping release of iOS on your iPhone, iPad, or iPod touch, you can simply install the final version of the software you are testing when it appears in Software Update. If you would like to go back to a previously released version, you must first erase the beta software, then re-install the latest shipping version of iOS, and finally restore from your previously saved iTunes backup.
Learn more about how to erase and install iOS
Learn more about restoring from an iTunes backup