From The P Systems Webpage

First, you have to to be registered as an author with edit permissions.

Being logged in, you have to visit the page that you want to edit. Then click Edit on the top of the page. Fill in / change contents. Click "Save page" or "Show preview" if you want to preview first.

If you want to create a new page, you have to make a link on an existing page. Then follow this link. There will be a notice that the page currently has no contents. Click Edit.

Fill in the new contents, then save as above.