How to migrate multiple pages in root directory into a new subdirectory?

I have multiple pages in the root directory of my website:
    
    $ pwd
    /home/doss/public_html
    $ ls
    a. mdwn    b.mdwn  section.mdwn subdir

What's the procedures to migrate a.mdwn and b.mdwn to subdir and keep correct links to them that might exist in other pages like section.mdwn?
