Conversion of .accdb files to .mdb basically represents saving latest Access databases to a format used in earlier versions. This is useful when you need your newer databases to be opened on computers with only old version of MS Office.
The most convenient way to do this is of course to use Access 2007 or older and use the File ► Save as.. function to produce a .mdb variant of the database.
You may also find some dedicated database conversion solutions that should be capable of the same, but it is always better to use the original software to ensure maximum compatibility.
Just keep in mind that because you are basically downgrading the format of the database, some features that were added post 2007 to Access may not always be successfully converted.