Created RDS db-instance Replica successfully, now how to connect it with server
What you are talking about is called Read/Write Splitting.   This is not a capability directly provided either by MySQL Server itself, or by RDS for MYSQL. It is an advanced technique for which there are no magic solutions, as I have mentioned elsewhere. 

If you are not modifying your application (and assuming your framework does not have such functionality built-in, which is a reasonably safe assumption), you will need to deploy one or more read/write splitting proxy servers to divert each query to the correct target as appropriate.  

Examples of software with this capability are MariaDB MaxScale and ProxySQL.