Software upgrade with vat inclusion

I need an Excel expert to add a VAT (5%) feature to my existing Excel-based accounting and inventory software.

Requirements:
- Apply VAT on multiple sheets
- Update totals while keeping existing functions intact
- Display VAT on each relevant line item and in a summary section

Ideal Skills:
- Proficiency in Excel (2013 to 2019)
- Experience with multi-sheet workbooks
- Attention to detail and accuracy

Please ensure the current functionality remains unchanged after the update.