In the two previous alliances I've been in, they rotated the alliance resource buildings. I'm now reading..

If you want to change the alliance resource building (Farm / Lumber mill / Steel / Gas Field) before the current one has run out then you need to view the details of the current one and use the demolish option to remove it, You can then immediately replace it with a new resource building from the alliance territory menu.

When you place the new building it will need to be built before you can start gathering. If lots of players send troops to it this will only take a few minutes.