Open the time entry you wish to edit. Then change the Client and Project as needed and click Save & Close.
The information will be switched from the first client to the second one. Note: This will only apply with a time entry that is not included in a finalized invoice. If the desired time entry is in a finalized invoice, just unfinalize the invoice, move the time entry to the desired client/project, and then go back and finalize the invoice again.