Am I elegible for an upgrade?

You can upgrade to whatever you want as long as you pay for it. Note the iPhone X is no longer available from Apple.

Xs, Xr, iPhone 8, and iPhone 7 are the current options.

You won't get a free upgrade to any phone however.

You can trade in your iPhone 6s Plus for a gift card you can use towards the purchase of any new iPhone you want.

The amount of credit you get for it, will depend on the state of your iPhone 6s Plus.

See here for details: Trade in with Apple GiveBack - Apple

Also, you posted in the Snow Leopard forum, which is a very old macOS version for computers, and has nothing to do with iPhones, so I have requested the forum admins to move this to the iPhone forum.

Probably not. The Trade in Program will give you a gift card with the value they determine for your current iPhone which you can use towards the purchase of a new device.

Look in the link, and follow the steps, to see how much you would get for your iPhone 6.

it's all in the link he posted