How can we access multiple openwrt routers from internet.
routers have private IP address configured.

First idea: Set up some VPN. Use a single public IP to set up a VPN server and connect all routes to that VPN. That single public IP can either be a cheap VPS in a data center or one router that has a public IP address.

But a better answer clearly requires more information about what you currently have and what you're trying to do.

routers will be installed at different location in city and i wanted to control is from central location.

which is the best, free (lifetime) and simple to configure VPN setup.?
Do we have any simple documentation to configure the above requirement,?

You most likely find OpenVPN to be the one with most documentation available and widely used.
Or you can use tincVPN which I prefer since it's damn easy as soon you realize how things work together.

Both of them are free.

But none of them are "services", you need at least one public IP the others can connect to.