It's a bit tricky (can lead to data loss). Consider backing up any important data and starting fresh (delete Ubuntu and reinstall). If you're still reading, these are the basic steps:
- Boot into a live session (Ubuntu live cd)
- Unmount and perform a filesystem check that resides on the partition you want to reduce
- Reduce the size of the filesystem with the resize2fs command
- Delete the partition's entry in the partition table with the fdisk command (or similar)
- Create a new, and smaller (exactly the same size as in step #3), partition entry in the partition table with fdisk (or similar)
- Mount the filesystem
When installing Windows, you will see free space. Tell it to use 250GB of it.