Method 1

1.  Unlock the JIRA Agile customfield to enable editing, see Unlock a locked Jira Software custom field
2. Rename the GH customfields, issue types, issue type schemes, workflow name and workflow scheme name/description from the UI
3. Perform an Indexing

Method 2

If and only if you have no valuable data on your instance.

1. Shut down JIRA
2. Remove the JIRA Agile jar file from <JIRA home>/plugins/installed-plugins/
3. Drop the database and recreate a new empty database
4. Start JIRA
5. Create a fresh backup after Starting JIRA
6. Reinstall JIRA Agile
7. Perform a restore on the backup created prior to the JIRA Agile installation
8. Verify problem resolved

Method 3

To search for Epic using the custom field name instead "gh.epic.link.name"

To search for Sprint using the custom field name instead "gh.sprint.customfield.default.name"