You need an account to edit the wiki (except for this page). There is a link to the registration form in the top-right. Once you have logged in, go to the page you want to edit and look on the right for a pencil icon. If you only see a magnifying glass, this means you only have permission to view the page. Get in touch at contact@greenbikeproject.net if you need to edit a certain page.
Once you have opened the edit page, you can edit the page in Dokuwiki format. Details of this are on the syntax page, but the key points are titles with '=' before and after the title (6 times for the main header down to 2 for the lowest level), double spacing between paragraphs and double square brackets for hyperlinks. If you look at the formatting for existing pages using the pencil or magnifying glass icon, you should pick up the basics.
Feel free to ask any questions you have about the wiki below.