Load balancer problem

I have 2 instances and i have established a load balancer successfully between them.
but i do not understand how to keep the 2 instances updated at all time with both their content.

The task of a load balancer is to switch to a different server instance if your previous server is down or loaded. You do not have to manually do anything to switch between servers. It takes the one which is more reliable.

To test, have different content to both instances and reload your website. You will see different content sometimes.