You can rename default groups through admin/customize/site_texts 
To change trust level 0 it would be groups.default_names.trust_level_0
It can’t share a name with an existing user or group, or the rename will not take. The change takes place as part of a background job, so will be updated when that runs. You can speed it along by triggering Jobs::EnsureDbConsistency from the /sidekiq/scheduler