database update control

I am creating a customised point of sale software for our company & we need to have a program that will do the database updates both to & from our twenty remote branches & head office. The databases are MySql & all branches are connected to head office via vpns over the internet. Each branch will have its own database. We need to have things like sales, stock locations, payments, credit notes, customers tables etc...updated across the branches & head office. The data updates do not have to be immediate but at least a few times a day. The POS software is 98% created & now we just need to solve the database communication / updates but i am on an urgent timeframe to get it completed.