[WhaLberg]

If you want to modify the rules page (which's before registering);
1) Go to your AdminCP.
2) Go to Styles & Templates.
3) Click to Style Manager.
4) Select your template.
5) Go to Registration Templates.
6) Select the "register_rules" template.
7) And edit it.

If you want to change the rules which is shown in that page;
1) Go to your AdminCP.
2) Click to Languages & Phrases.
3) Select Search in Phrases.
4) Paste that "forum_rules_description" to Search for Text field.
5) Go find "Phrase Variable Name Only" and choose it.
6) Then click find.
7) You will see the rules, click edit.
8) Edit the rules.
9) Save it.
10) That's all.