How to update Value Map between two Standard Lookups without a Sandbox

Summary: Need instructions on 'how' or 'if' there is a way to update the Value Map between two Standard Lookups without creating a Sandbox.

Content (required): Currently have two Standard Lookups with a value map between them. I can update the value map by creating a Sandbox and updated via the field using the Standard Lookup, but would like to know if there is an API or UI options which could be used instead.

Version (include the version you are using, if applicable): n/a

Code Snippet (add any code snippets that support your topic, if applicable): n/a