Hi Rahul,
Seems like to new to scn.
Welcome to SCN.
There are lot documents which are related the same.
always try to search on google with similar words. you will get what you need.
Use search term " RFC connection ecc to bw" or "changing ecc source system in bw"
its basis activity, take sap basis team help.
Useful t code - WE20/21, BDLS.
Thanks