Once you have the rights and that you are logged-in, the "Edit Content" button in the menu allows to switch to edit mode. This button appears on the top left if you have the rights to edit the page and are identified.

You will find information about the XWiki syntax on the page XWiki Syntax.

Once your document has been modified, you can preview it by clicking on the "Preview" button, or directly save it by clicking on the "Save button". In the "preview" mode, don't forget to save your document !