Hello @richb444 ,
The quickest way to update your existing contacts' fields in one go would be to utilize the type/paste multiple or file import methods of importing contacts. Any existing contacts that would have new information as part of the uploads will have the old contact fields overwritten.
For example, if you import a CSV file with just the email addresses and first names, any contacts already in your account that were re-imported with a new first name will have that info updated. Anyone else who still had the same first name field, and any fields not included in the re-import, would stay the same.
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