Thanks for reaching out about this!
Unfortunately at the moment it isn't possible to change a Custom Type's API ID:
If you really need to change it, you will need to create a new Custom Type with the correct ID. Then you can copy and paste the JSON code to recreate the fields. Then all you need to do is recreate all the documents of this type.
You either need to do this manually or you can use the Import/Export feature to help. This feature is available starting with the Medium plan.
What you can do is to change a Custom Type's display name:
When you do this, the display name used in the Prismic UI will change everywhere. There are no risks to doing this.
I'll add this thread as an open feature request.