The easiest way to convert mdb to accdb, i.e. conversion of different versions of MS Access databases (.mdb and.accdb files) from older format to newer one, would be to use latest Microsoft Access, because only these support both of these file types reliably. Access prior version 2007 did not use .accdb file and hence cannot save or open it and thus cannot be used to perform mdb to accdb database conversion.
You may also find some other 3rd party database converters that support Access formats or even dedicated Access conversion tools.
Updated: June 21, 2023