A group of Bitcoin companies or exchangers like coinbase , ZebPay, Coinsecure, Unocoin and many more decides to deploy a hard fork to double Bitcoin’s block weight limit to eight megabytes this November. Known as “SegWit2x,” this incompatible protocol change follows from the New York Agreement (NYA) and is embedded in the BTC1 software client. Although, if an organisation who is part of Bitcoin network doesn’t want to accept the forked currency they can continue with legacy one(i.e., Bitcoin).
Yes, you read that right, exchangers or anyone can make a decision to upgrade the protocol and make his own network. But it’s success depends on the cause, the benefit of the upgrade and adoption by the crowd.
But if you want to upgrade, say BTC, you have to talk to the Bitcoin Core Team. But if you don’t want to do that, or want to make the upgrade even if the core team don’t agree, then hard forks happen, and new network forms (Example Bitcoin Cash, Bitcoin Gold, Segwit2x and so on)