How can I change the default google account in google chrome?

Since Chrome 69 any login to a Google app causes an automatic login to the Chrome Browser with the same account.

There has been a strong protest on the Internet, and Google has promised to add an option for it on Chrome 70.

Until Chrome 70 comes out, and as long as this workaround still works, you may open chrome://flags/#account-consistency, find Identity consistency between browser and cookie jar and set to Disabled.