Change Sales Order Tax Classification Code

Dear Sir

It is because the VAT tax changed from VAT Tax 16% --> VAT Tax 13%. We would like to change tax classification code discription from VAT 16% to VAT13%. Do you mind to let us know how to change this information. We are added VAT Tax 13% on Tax Managment.We checked that system is already using VAT Tax 13% to calcluate in line by line. it show cannot disable CN VAT 13%. Can you help me know what wrong on my step?