The procedure is:
1. In ACP, disable all the extensions. (ACP >> Customise >> Manage extensions)
2. Set the theme to prosilver (ACP >> Customise)
3. Check that language is set to British English (ACP >> Customise >> Language packs)
4. Then log out of ACP.
5. On the server, using FTP, delete all files on the server except for /ext/ /images/ and /files/ and config.php file (leave these on the server untouched)
6. Download a copy of your forum logo and keep it it to one side.
7. Download a new install of phpBB from here ->
https://www.phpbb.com/downloads/
8. Unzip the installation files.
9. Delete the blank config.php file, the /ext/ /images/ and /files/ folders from the unzipped files.
10. Upload to the server keeping the file structure
11. Visit
www.yourphpboard/install/app.php/update
12. Hit submit
13. Select "Update Database only" and hit submit
14. You get "The database update was successful", log in to ACP and enable your extensions (ACP >> Customise >> Manage extensions)
15. Install your theme.
16. Set your forum logo with the one you downloaded in step 6