From today you can domwnload and install test versions of iOS 10 on comatible devices. You must only enroll device to tests here.
Here you have list of devices that can install iOS 10:
iPhone 6s
iPhone 6s Plus
iPhone 6
iPhone 6 Plus
iPhone SE
iPhone 5s
iPhone 5c
iPhone 5
iPad Pro 12.9-inch
iPad Pro 9.7-inch
iPad Air 2
iPad Air
iPad 4th generation
iPad 3rd generation
iPad 2
iPad mini 4
iPad mini 3
iPad mini 2
iPad mini
iPod touch 5th generation
iPod touch 6th generation
Sierra is also avable for public tests on same rules as iOS 10.
For final word I must inform you that this are test versions whot means there can be problems with stability etc. and you souldn’t install those on main devices and if you do it on your own responsibility.

Source: gsmarena