Why doesn't my Lumia 640 update to Windows 10?

[Windows Central Question]

I have tried updating the phone manually, but it says that my phone is up to date. It isn't - device info says I am using Win 8.1 Update 2 (v. 8.10.15148.160). I have 2GB free memory. Why doesn't it update itself to Windows 10?

 

[arvind gupta1]

They haven't released it yet... The preview can be made available by downloading the windows insider app from store...