How to force install Windows 10 technical preview to my laptop?

[iice cage]

Since the windows 10 came out, I successfully installed it to laptop several times. Because my laptop is non pro windows 10 technical preview. I always ask how to get pro version of Windows, and accidentally installed a windows 8.1 pro with media center product key. I noticed that this is just a evaluation copy and already expired. Now my laptop too slow and restart every time. Now When I run windows 10 TP SetUp, it always failed to install. The set up says, " we can't tell if your system is ready please try again", can you give more tips to install windows 10 TP to change the expired evaluation copy of product key?

 

[hotphil]

Do a clean install from ISO - format the machine.

 

[iice cage]

Now my laptop doesn't start but on just blue screen with RECOVERY and more options """can't verify the signature or """ it seems like that. I have many importants files from my C drive can I backup those files before clean install? Thanks for the reply.