Lots of options to trade in your current iPhone 6s Plus and buy a new iPhone 11 Pro Max. Here's one way do it through Apple. No in person-visit required. As described in the linked article, you can buy the new phone online, and get a trade-in mailer sent to you. You just send in the old phone using the mailer and otherwise follow the needed steps to keep wipe the old phone after you've transferred it over to the new, so as to keep your data safe, etc, and, in the words of Ari Gold, Boom! New phone in hand and old phone traded in.
There are lots of other options if you search, but I personally like the simplicity of the Apple approach, even though it may generate slightly less trade-in dollars.
Happy trading! :-)
-- JDee