Once you select the time entries for editing, you can click on the Edit link in any time entry report.

The changes you make on the bulk edit screen will impact all the selected time entries on that page.
You can select EITHER the checkbox options or the 'Update bill/cost rates to latest' option on the edit screen.
Selecting the Update bill/cost rates to latest option will fetch the latest bill/cost rates for the selected time-entries. This is helpful when your rates change and you want to update the rates to the latest for the existing time-entries.
Selecting the individual option (checkboxes) is helpful when you want to specify the rates/date, etc for select time-entries. For example, if a timecode changes from non-costable to costable, in such cases, you can select the time entries you want to mark as costable and then specify a rate to apply.
Even if there is one time entry that can cause conflict, an appropriate message is shown. Till you un-select that time entry, the changes cannot be saved. For e.g you are updating a bill rate of some entries, which include a time entry which is non-billable. In such cases, a message will be shown. You need to un-select the non-billable time entry and then edit the time entries again.
When you specify a rate or update the rates to latest, this will affect time entries in all states, including Invoiced provided you have the appropriate permissions.
Non billable/non costable time-entries need to first be made billable/costable and then specify a rate for them. Only specifying the rate will not suffice.
You need appropriate privileges to perform bulk actions. For details, refer here.

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