Update Header with summed detail

I have a header table with vendor number, invoice number, invoice date as the key fields and invoice amount and tax amount as data fields.
I have a detail table with vendor number, invoice number,invoice date, account number as the key and account amount as the data.
I want to update the tax amount in the header with the results from the following query:SELECT tbl_Inv_Detail.Vendor_No, tbl_Inv_Detail.Invoice_No, tbl_Inv_Detail.Invoice_Date,
SUM(CASE tbl_Taxes.Type WHEN 'G' THEN tbl_Inv_Detail.GL_Amount ELSE 0 END) AS GST, FROM
tbl_Inv_Detail LEFT OUTER JOIN tbl_Taxes ON tbl_Inv_Detail.GL_Account = tbl_Taxes.Tax_ID GROUP
BY tbl_Inv_Detail.Vendor_No, tbl_Inv_Detail.Invoice_No, tbl_Inv_Detail.Invoice_Date
I have been trying to find a way to code this so my query would look something like this:
UPDATE tbl_Inv_Header SET tbl_Inv_Header.Tax = GST FROM (select query )
but I am not completely sure of the syntax. could some one help?
Thanks,
Tom

Thanks,
That worked