There is no quick way to update tax code rate changes, unfortunately. The best practice is to use tax classes for all sales - sales parts, sales objects, report codes etc, and then the tax class can remain the same but the tax code in the class can be updated.

There is no quick and easy way to update the purchasing side - supplier and purchase part setup has to be changed either with a migration job, script or manually by changing the tax code in the setup. If you are not currently using tax classes for sales side, then that has to be done through the same update.

We suggest not to change the tax rate for the existing tax code but rather add a new tax code because that could cause issues with existing transactions.

Supporting IRituma’s answer, I would like to highlight that if you change tax rate on existing tax code will not allow you to register properly delayed supplier invoices (having “old” tax rate applied). You will have problems as well if you are using tax due at payment. 

Creating new set of tax codes should be the only option considered.

Please also keep in mind that adding the tax code in the tax class does not update already existing orders and you need to have an eye on newly  created invoices. 

In general many data sources need to be considered when using scripts for example:

just to name some: Customer Order Lines, Customer Order Lines Tax, Sales Parts, Customer Order Charges, Order Quotation Lines, Quote Line Tax Lines, Identity Invoice Info, Purchase Part Supplier, Customer Info Vat, Activity Cost