If you're looking for how to change a rate for specific user, check out this article.
The rate of your current matter is incorrect, and needs to be updated.
To get started, navigate to the Matter in question, and open it's Access and Billing tab.
Once you save the changes, you're give the option to update previous entries to the new rate.
If you "Update Entries", all previous entries for the Matter will be updated to the new rate.
If you "Don't Update Entries", previously entered entries will remain at whatever rate they were originally created with.
You can also change the rate on specific line items in Draft Invoices. Open up the Draft, and click on the rate:
Once there, go ahead and edit the Rate for the specific line item.