Allow entry in Browse mode in the field.
Show all records.
Sort them if you want.
Go to the first record, tab into the serial Number field.
Choose Ctrl (or Cmd) + = (or Records -> Replace Field contents) and select the 'Serial Number' option, starting at the number you want.
*** READ THE DIALOGUE CAREFULLY. THERE IS NO UNDO FUNCTION FOR THAT STEP ***
Go to 'Manage -> Database, choose the field, and set it to auto-enter a serial number. Set the 'Next value' to be the correct next value.
You may wish to remove entry in Browse mode now.