call 121. go through the menus to eventually connect to a human being.
tell him that you are not happy with your plan. and you are considering switching to some other service provider.
at this level they are likely to only offer you official plans. tell them that you want to disconnect.
at that point of time, they would either send you to retention team or tell you to wait for a confirmation call.
in either case... do remember that they might not offer you anything good and might just directly disconnect.
though you should get 2-3 calls to confirm before that happens.
Regarding your vote, I recommend you stay with airtel. I have had some horrendous experiences with Reliance. They still owe my brother 1000 when they suddenly went bankrupt and turned off reliance broadband. Don't expect anything different from other ambani. They might offer magic speeds but it's all about reliability and customer service which they lack. Airtel has been transparent in their service. Good luck.
I got my unreliable Airtel connection disconnected recently. Have been trying from past 15 days to make payment. Just does not seem to work. Now they are saying that you have to come to our outlet and make payment. Bunch of losers. This is 2018 and they don't have facility to clear the final bill online. What a joke.