Before you delete custom fields that will affect all contacts who have that field, consider this alternative!
If it is simply your goal to change the data in a custom field and have it apply to all the relevant contacts, you can take the following actions:
1. From your Contacts tab, create a Segment to search on your custom field. To learn more, please see our Segmentation support article here
2. "Select All" to export these contacts. Your custom fields will export too.
3. In the CSV file, remove the unnecessary fields or change your custom field entry to whatever you desire.
4. Re-import the file into Nimble, and select "Override existing contact fields with data in the CSV".
Please note, if the custom field you are changing is a "select box" custom field (the Lead Details fields are an example of this), you will need to ensure that the dropbox values exist in Nimble in Settings >> Data Fields before re-importing your file to Nimble.
Now all your contacts with the custom field will set to the new values you created in your CSV file!