Open Boot Camp Assistant again, then click Continue.
Just select the option to "install Windows … or later version."
Click Install, then follow the onscreen prompts to repartition your drive and install Windows.
When you complete the assistant, your Mac restarts to the Windows installer. When you're asked where you want to install Windows, select the BOOTCAMP partition, then click Drive Options and format your Boot Camp partition (follow this link to see how).
Follow the onscreen prompts to finish installing Windows.
You're ready! Windows is installed on your Mac. Well done!