Previous behavior
The percentage of a tax could be changed after it was created. When the change was saved, all transactions - past, current, and future - with that tax applied would be updated to show the new rate.
New behavior
The tax rate cannot be changed once a tax has been configured and saved.
If a new tax rate is required, you can create a new tax with a new rate.
This new behavior preserves the integrity of existing transactions.
To view your current tax rates, go to Settings > Finance > Apply taxes on transaction categories.