DBASE Software coding amendments

I have some software that appears to be written in DBASE in Dos and requires two or 3 small amendments (such as Telephone Number, VAT number, VAT Rate (for VAT calculations) etc) as this seems to be hard coded in the software and needs to be amended and recompiled into an EXE. The software was originally written around 1985. but as it still works fine, I want some amendments to company details as these have changed over time. The VAT rate is the main thing that needs to be changed.