Trades. I can change a trade offer by adding a different resource but some players can eliminate a resource and add a different one, How is this done?
When you have the trade screen open, clicking on the top or bottom of a resource card will add or subtract that resource from the trade offer.
If the player offers wheat for wood and you want brick instead of wheat you can subtract the wheat and add brick in the upper row.
If you want the wheat, but would rather give them rock, you would subtract the wood and add rock in the lower row.
There's probably a video on YouTube which explains it better.