NOTE: Before proceeding further, please make sure to have a working backup of the database.

1. In SO Administrator, connect to the database. (File> Connect)
2. Click on Query and select New.
3. Copy and paste the following:

Select TaxID, Description From Item

4. Click F5 to run the statement.
5. Please take note of what number is assigned to the TaxID of the items.
6. Copy and paste the following:

Update Item Set TaxID=x Where TaxID=0
NOTE:
x=the value noted on step 5.
This statement will update all items that do not have any taxes set.

7. Click F5 to run the statement.