Give read-only member read-write on single sub-folder?

I have a Sharepoint site (via Office 365) called “Public Documents” and within there I have a COOP folder. Up until now all the regional members of our organization are only supposed to have read-only for the top-level Public Documents and all sub folders.

A ticket just came in that I need to give one single user read/write access to the COOP folder only (Public Documents > COOP). How can I do that without affecting other users permissions? This user “Jane” is a member of the read-only group currently.