Hi David,
- Go to Settings > Site administration > Language >Language customisation
- Select English in the dropdown menu, then click on button "Open language pack for editing" and wait for loading, then click on "Continue"
- Select all files in the box "Show strings of these components" (unless you know the exact component where your string lives) and type "Surname" in box "Only strings containing"
- You'll get 4 strings where you can change what you want in "Local customisation"
- Click on "Save changes to the language pack".
That's all folks