Pastel Xpress Start-up "allow tax" for inventory

[Hannerie]

I have been using Pastel Express Start-up for a year now. I have recently been registered for VAT. how do I change the entire inventory to "allow tax" when it previously did not, and also to change the tax types for selling and Purchasing.

I have tried to export the inventory to Excel, changing all the lines and then to import back in to pastel. But somewhere something goes wrong and nothing is imported.