It is not possible to switch an existing timesheet from one client to another. The correct process is to Credit the invoice (if you have issued it/exported it) or Void the invoice (if it has not gone anywhere yet) and those timesheets will revert back to where they were validated (if you choose to Void the invoice the timesheets will revert back to the Create Sales Invoice area).
Next you should create an assignment for this client and candidate arrangement in order for correct timesheets to be generated against the client. If the client does not exist you will need to create it first in the Client Entry area.
Once you have created a new assignment, go to the Incoming Images area and remove the image(s) from the original timesheets - you can do this by setting the processing Mode to Add Image, locating the timesheet(s) then clicking here to remove the image(s):
Those images will be placed back in the Incoming Images area, ready to be used again. To validate the new timesheet for the correct client with the original image, click to select the image in the queue, ensure you are in Validation mode, select the Candidate then select the correct timesheet:
You will then need to enter in the units onto the timesheet and validate it in the usual way. The timesheet will then go to the Create Sales invoice area ready to be invoiced.