Both .mdb to .accdb are databases from MS Access, so the easiest way to achieve mdb to accdb conversion (conversion from older to newer format) is of course to use the original program in version 2007 or later, because previous version did not have ACCDB format. It is likely you can also find some 3rd party database converters.
Editor of Microsoft Access databases for Mac
A database viewer
Management tool for Access databases on Mac
A powerful database to database converter
A relational database management system from Microsoft Office
Fully-featured version of Navicat for Windows