You can change your hourly rate and select if this change will apply to all unbilled activities related to this user.
In the Settings>"My profile", click on pencil icon in the Information block.
Change the hourly rate and activate the below toggle, if you want this hourly rate change to be retroactive for all past unbilled activities related to this user.
If you choose YES and set up a date, all the time entries created after this date will be updated according to the new hourly rate.