You have the ability to change the citation style used when exporting from Authorea by navigating to Export -> Export Options, then selecting your preferred style as shown in Fig. 24.  Note that the references in your working document will not reflect this style choice.  As mentioned above, they will retain a generic Authorea style.  Formatting occurs only when you export the document, so be sure to check the reference section of the export to ensure it reflects the style you specified.