While we don't currently offer the ability to bulk update contact fields from within Nimble, you may update fields by uploading a CSV file to Nimble.
Open the CSV Import Wizard and add your file to update contacts.
When uploading choose from any of the following merging options which are available on the third page of the import process.
Overwrite data from CSV - Replace Nimble data with data from the CSV file. Won't touch fields that aren't part of selected mapping
Replace contacts with CSV - Replace matched contacts with CSV data. This is helpful when you want to start fresh with a group of contacts. It will wipe out all the data on the contacts and overwrite it with the CSV data.
Skip contacts with merge conflicts - Skip contacts with conflicts. Select this option when you don't want to lose any data currently in Nimble. e.g importing your LinkedIn contacts or other data files where you can't guarantee that data is better than what currently exists.
For a full walkthrough on importing your CSV file, take a look at the following: