How do I rename an existing fortress?
According to the Dwarf Fortress wiki:
You can choose the name of your own fortress during the Embark process. When you reach the stage where you are deciding on your founder's skills and starting items, press Shift+F to choose your fortress name. You will be given a semi-random name automatically. Fortress names are always compounds of two dwarven words. If you choose to pick your own name, you can craft a fortress name that would otherwise be rejected by the name generator.