1.  To use a different source to cite your idea or to add additional in-text citations to the same idea, click on the citation you want to change, then choose the Add/Edit Citation icon. 

Next, enter in the information about the new source into the red search bar.  Choose the new source and delete the unwanted source, or keep all of the sources you've selected in the red bar.  Your changes will also now be reflected in your bibliography.

2. If the type of edit you want to make is due to an error you have found in the citation, it is best to make this change in your Zotero library.  This way the correction will remain in place for any future papers you may write that use these sources.  Simply go to your Zotero library, and change the incorrect field.

3. To make a change in your library go into effect, click the “refresh” icon.  Your bibliography will also be updated.

4.  Sometimes you may need to make edits to your bibliography, using the edit bibliography icon.  
The most common reasons I am aware of to make changes to a bibliography are to italicize scientific names and to insert sub or super scripts. 

5. Select the reference you would like to edit, then use the rich text editing tools to make your changes.  You may also add additional references to your bibliography that are not listed in your in-text citations this way if you need to. You can simply make the edits within Word; however, any edits you make within Word won't show up in your Zotero library (i.e. Zotero talks to Word, but Word doesn't talk to Zotero).