Don't go change any original entries and then have to re-total your book. You will also not be able to recreate your book unless ALL of your CFI's are readily available to redo endorsements and re-sign dual flights.
Like blastoff said, do "journal entries"...find each error, and then for each error make an entry (today's date) correcting the time (might be a negative value) and in the comments put a note to explain the correction. Ex. "Corrrection for Math Error on 5/7/2007"
I had a few of these to do myself after I transitioned to an electronic logbook
