how to put apple watch in demo mode

You can't. Demo mode is not something you activate. Its a special version of the watchOS software that is installed on watches meant for store showrooms only so they cannot be accidentally locked or disabled.

Why do you want demo mode?

There is no "demo mode" on apple watches sold at retail. The only watches with a 'demo mode' are those specifically designated as demo units for use in Apple Stores and some retailers. They are not available for sale.