you can delete the accession number by opening the holdings of the title
technical processing -> search catalogue ->
then search the catalogue(using title or acc.no)
select the title click on update holding then it will show the accession numbers which or belongs to that title. select the particular acc.no which you want to be delete. then right side you have delete option.
but weededout/checkout books can't be deleted only available books can be deleted and more over.
better to redo the weededout then delete the acc.No