I want to change here the kubuntu tag to ubuntu. But i could not find it.
Is it lost? Or I'am blind.
If you are posting on Ask Ubuntu, it's implied you're using Ubuntu or an official variant of Ubuntu.
That's why we don't have an ubuntu tag - it's superfluous. Use the version tag to identify your version of Ubuntu if the question is specific to that version, and only tag variants of Ubuntu if you are not using Ubuntu Desktop or Server - otherwise it's implied.