To edit a Wiki page, you first need to log in. See Signing Up And Logging In for more information.
You should then see a tab that says "Edit", either just below the page title or at the very bottom of the page, depending on what theme you are using. Click that to enter the editing interface.
You should be able to mostly just type whatever you want, but if you want to do anything complicated, either click the tab in the editing screen that says "Wiki Quick Help", or read WikiSyntax for more information.
As a stylistic note, since this Wiki is meant for content related to Lojban, and Lojban makes very little use of capital letters, and in fact tends to look ugly if you capitalize it, we'd like to standardize on the use of double parentheses, (( )), around links.