You can achieve this by using CASE as showing below.
SELECT Tax_Code.TaxCode_Id, Tax_Code.TaxCode, Tax_Code.serviceTax, Tax_Head.TaxH_Id, case Tax_Head.Type when Tax_Head.Type='U' then 'label1' when Tax_Head.Type='P' then 'label2' when Tax_Head.Type='O' then 'label3' when Tax_Head.Type='I' then 'label4' else '' end, Tax_Head.Project_Id
FROM Tax_Head INNER JOIN
Tax_Code ON Tax_Head.TaxCode_Id = Tax_Code.TaxCode_Id INNER JOIN
TaxMst ON Tax_Head.Project_Id = TaxMst.Project_Id
WHERE (Tax_Head.Project_Id = @ProjectId)