How can you convert a bike to hybrid bike?

Depends on where you're starting from. A drop bar road bike can be made a bit more user friendly by switching to a flat handlebar, and some slightly wider tires. Do note that you may end up having to get new shifters/brake levers to go on the flat bar.

Also note that not all road bikes are able to use wider tires, Check fork crown and chain stays for clearance.

If you're starting off with a MTB, start by replacing knobby tires with narrow, smooth ones. Next step is suspension. If you have a sus fork, replace it with a rigid one.

If your bike is full sus, sell it and buy a hybrid instead.