You can make a copy of a bib in Horizon by having it open in the MARC editor and clicking on File - Copy Record. That will create a new record. If you look at the tabs at the bottom of the screen you will see the original record with the bib number and the "New" record in another tab. I always close the old record - just to be safe so I don't get confused about which record I am working on!
Make sure you review/change the following:
001 - control number - delete
020 - delete those not on your book and add your isbn
100 - primary author - make sure it matches your item
245 - statement of responsibility - make sure it matches your item based on the Title page of the book
260/264 - publishing info- make sure it matches your item
300 - extent (pages, illustrations, size) - make sure it matches your item
250 - edition statement - make sure it matches your item
945 - bib number from old record - delete
If you are copying a record that needs to be changed from/to a different format such as an audio book or DVD to BluRay there are more fields to review.
000 Type - needs to be changed
007 - added for audio/video materials and removed if changing to a print book
008 - some fields need to be reviewed - they will be in red after you make changes to the 000 and 007 fields
336/337/338 - need to be updated
5xx - notes - make sure any notes match your item
7xx - creators - may need review for narrators of audio books