How to restore Skype history?

You cant restore skype history.

If you're talking about your chat history, generally once deleted, its gone for good. However, do you have any full system backups? You may be able to restore an old Skype file from a few days/weeks ago depending on your backup schedule.

Skype is a sort of 'first come first served' and is free as you know, so I doubt an old skype file can be retrieved.

Have faith tickle. Anything is possible in the digital world.

Who knows... maybe next Microsoft will buy Skype and all your chat history will be saved forever in the "Cloud"