Search function is a very handy tool. Below is the link to the tutorial on how to setup SQL Express and how to convert a CE database. I recommend SQL Express 2016 it is the latest version. The setup is the same as 2014 so the tutorial is still relevant.
Once you get it setup and connected you can follow the procedures to convert your CE database and import it into SQL. Once you finish that you will need to open SambaPOS and go to Manage > Settings > Program Settings > Maintenance and run the maintenance task for Upgrade Old Rules and the various other tasks.
All that said it is not recommended to try and upgrade from V3 it is best to install SambaPOS into Fresh Database and setup your rules and automation manually for best experience. Although that is a lot of work.
