If the standard VAT/GST/Tax rate used in your country changes, then it is always recommended that you add a new rate rather than editing an existing rate. In some cases, the tax authority will expect you to apply the old rate for credit notes, refunds etc.
Under no circumstances should you change the rate of any existing Tax Rate codes. These are not date delimited and changing them could result in incorrect tax being calculated on older transactions that you edit.
There are a number of areas that Tax Rates might have been assigned and you will need to change this yourself as appropriate:
In addition, please check the website of your tax authority to understand the specific rules on managing scenarios that arise from a rate change, e.g.: