The text above the login form is stored in
inc/lang/en/login.txt. To modify you can copy it to
conf/lang/en/login.txt. Putting your own translations in the
conf/lang/ keep these changes on upgrade of the wiki software.
The strings below are stored in
inc/lang/en/lang.php with the keys: 'reghere' and 'pwdforget'. But you get than also the default links behind them to DokuWiki internal register/reset pages. Not sure how you replace these links. You can disable these via https://www.dokuwiki.org/config:disableactions (if you like to modify the text, you can follow this example https://www.dokuwiki.org/localization#example)
I see that dokuwiki.org has custom text and links below the form. But I don't know were that was done.