There is no easy way.

1. At the top of your page 3, before any text, insert a Next-Page Section Break.
2. Go into any headers and footers in the new section and tell them to not link to previous.
3. Still in the header (or footer), under Page Number > Format Page Numbers, tell it to start page numbering at 3 rather than continue from previous section.
4. Go to the end of Page 2 following any text and insert a Next-Page Section Break.
5. Paste your new page 2a.
6. Go into the header and footer and make sure they are not linked to previous.
7. In place of the current page number (3), type 2a.

This is a very poor way to organize a document.


Sections / Headers and Footers in Microsoft Word 2007-2013