I would like to remove ubuntu 14.10 on my computer. It will be installed as alongside with window 8.1. How can I remove it without window 8.1?
You can safely reinstall Ubuntu without affecting Windows installation.
During Ubuntu installation, you will be able to choose between deleting Windows and doing something else. Go to something else option and point installation to a partition you already had Ubuntu at. You Windows won't be touched at all.
If you need more details, take a look at this thread