There are two ways to do this:

1. Open the City Details. Look in the top left corner. There you can see the icons of the current enchantments. Each icon has a small X in it. Click on the X and it will remove the enchantment. Now you can cast another enchantment on the city.

2. Click on the Mana in the Resource bar at the top of the screen. This will bring you immediately to the Enchantments tab in the Ledger (quickest way to go there). Now, search for the name of the city. Then choose which enchantment you want to remove and click on the X on the right side of it.

Hope this helps.