Airtel — airtel conversion of post paid to prepaid

Address:

518543

I am a post paid customer and have been using airtel post paid connection for last 3 years
I approached airtel store to change they are not changing the number
Saying your payment bill date 21st come after 21st
After that again i meat next month they saying machines are not working come next day
Next day i reached office they say that payment date 21 st come next bill date ui told yesterday i came you guys only said to come today then they said whatever we don't know we will not change from postpaid to to prepaid. Come next month 21st

This type of nonsense they are doing since last 3 months

I got frusted with them

Please help me to resolve y issue