Need to reformat an invoice in MS Word into a template.

The attached invoice is generated by our accounting software and its format is too complex for me to edit and change the date that I need to change. For example I cannot simply delete a line item, or a number without messing up the entire document. The deliverable will be an invoice that look exactly like the one I am submitting, but that has number fields easily changed, line items that are easily added, modified or removed without messing up the format, name and address fields, etc. I want to be able to create custom invoices, where I am inputting the data directly into the invoice template.