If you are familiar with the Wiki format, you can make any changes in the manuscript using the Wiki editor ('Edit' menu option) or the Wiki visual editor ('VisualEditor' menu option). These are usually the preferred options in case of minor changes in the manuscript.
In the case of major changes, it is probably easiest to make the necessary changes in the original Word or LaTeX document and then reimport it, by using the 'RE-IMPORT' option of the right hand side panel.