I messed up my main table and need to update the values in one field back to the original copy. The backup is identical save original data in the messed up field.
Presuming you have a usable backup, you might be able to import that table from your backup copy to a WORKING copy (which will lead to having a total of 3 copies involved, and DON'T TOUCH THE ORIGINAL until this is done.)
Import the "clean" version of the table from your backup to a new table (different name) in the working copy. This step can be done from the External Data tab. The dialog will ask for a new table name, and to be honest, the name is immaterial. Just pick a name. At this point you will have the clean and dirty table copies side-by-side in the working database file.
Now manually create a query that looks like the UPDATE query shown next, substituting the correct table and field names, using the query builder in SQL mode. I've spaced it out a bit for readability.
Code:
UPDATE MainTable INNER JOIN CopiedTable
ON MainTable.Primekeyname = CopiedTable.Primekeyname
SET MainTable.ScrewedupField = CopiedTable.ScrewedupField ;
Once you run that update, open the table in Datasheet View and CHECK the contents. Do NOT assume my instructions were perfect or that your implementation was perfect. If it doesn't look right, then either you did something wrong OR your description to me was misleading. But since you are working on a copy, you can delete the copy and start over again.
IF the updated table looks good, then delete the arbitrarily named table you copied from your backup copy. (Delete from the working copy.)
Then do a compact & repair on your working database.
Then rename the original DB file to some other name, perhaps by changing it from Name.MDB to Name_Bkup.MDB. Or something simple. You will want to keep this copy around for a couple of days just in case.
Then rename the working copy to the original name.
Do this carefully, one step at a time. When the renaming is done, try to use it and see if the updates make sense.