You can remove a previously built piece by clicking the delete icon (top right), then as you move the mouse over the image the piece to be deleted will be highlighted in red, and then double clicking will remove it.

If an upgrade is highlighted in green, this means that building it will unlock further upgrades (these might be completly new pieces, or additional styles for that piece). Replacing a piece in this way does cost an upgrade point - your people won't work for free! You can change the style of a previously built piece in the same way, right click till you find the style you want and left click to build.

Some pieces have multiple styles that you can choose from, a messgae is shown when this is the case, and you can cycle through the available styles by right clicking. Left clicking will then build the selected upgrade. Building UpgradesIf you have upgrade points available, moving the mouse over the image of either the palace or the throne room will show what upgrades are possible.

Civilization 2 advisors