You would request an alias and wait for it to be implemented. I recommend this method due to the number of posts for that character.
If you wanted to do it yourself, you could mass edit it manually by going to Tag Script in the drop down menu beneath the Search bar, typing in '-yatogami_tooka char:yatogami_tohka' (minus apostrophes) in the new box that appears, and then clicking every image with the yatogami_tohka tag to change it yourself. It doesn't have a wiki, but if it did, you need to move the contents of the wiki from the old tag to the new one.