Microsoft — Lumia 540 unfair charge

Address:

Kolkata, West Bengal, 700099

Hi, i have lumia 540 and after updation to windows 10, it showing battery issue. I visited one lumia care to check the issue, they told either they will downgrade to windows 8.1 (Which will charge 450) or change the chip (Charge around 2k). I asked them take the phone check the issue and after than do the needful, but they told they can either downgrade or change chip. As mentioned in varios microsoft blog i asked them to downgrade to 8.1, but i can see still same battery issue. My question is why i am charged 450 unnecessary and how to resolve this issue quickly.
Waiting for quick response