The setting is in: Administration Center > Features > Sales.
Default sales tax / VAT (text products only) option set that your products that were added as the text will have inserted default VAT rate when you are creating a new bookkeeping record. The value can be changed during the bookkeeping record creation.
Click Default sales tax / VAT (text products only) to set its value.
You fill in the number that represents the VAT rate in percent.
When you change the settings, be sure to click Save. For more information, go to Features.