Why can't my Lumia 620 install the update to Windows 10 Preview builds 10508 the latest?

[Windows Central Question]

my lumia 620 cannot install update to windows 10 preview builds 10508 the latest

after downloading of 50% of the preview build it requires a restart i choose restart now button,then the phone shows only installing update like two setting's tire rotating on black screen suddenly it shows a emoji like smile or something then again restarting it continues what will i do ,,,,pls help me