You can create a new fee schedule each year (based on an existing one) with updated rates for all the relevant resources. Then associate that fee schedule with the projects that you want to bill using the new rates. This way, you maintain old rates for existing projects where the contract terms require. Check CORE Help Center for details.
You can also change the default bill rate for each employee, which would then affect all projects, but only for new time entries, not existing ones. It will not be applied retroactively to older entries. To do so, you would have to manually edit existing entries using the Actions > Batch Update option in Time Entries after filtering for your time frame. Check CORE Help Center for details.
Any new entry will pick up the new bill rate, regardless of the date for which time was being entered. In other words, the rate change is effective from the time it is entered; so if you changed it today, the new rates are applied from today moving forward.