How do I edit references and present them in author or year only form?

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Citation modifiers such as suppressing the author or the year in the in-text citation are available in the Edit Citation context menu option when you right click an in-text citation.

You will find the citation modifiers in the "Show" dropdown menu in the citation editor popover:

See also http://support.manuscriptsapp.com/topic/115/citing-with-a-page-number for citing a specific page or page number range (the screenshot above is outdated in that the control for page range is actually in this same popover).