You ca't change it, but there is a temporary solution. If you have an extra wiki made before March 11, 2020 that you don't use anymore, you can contact FANDOM Staff to change the name for you so you can write on that wiki instead.
That would be pointless. Wikis that use the legacy platform will me migrated to UCP eventually; likely by the end of the year. The only thing this would accomplish is delay the use of UCP at the cost of losing an existing wiki.