Step 1
Select the contacts you want to update.
Step 2
Be sure the "country" field is included in your list view as one of the columns, then export the list.
Step 3
Update the values in the country column in the exported list to use a supported ISO country code or country name.
Step 4
Remove unrelated columns from the file - leave only email and country.
Step 5
Re-import the list using the following settings:
no need to add a new interest/list
use a source "Existing contacts"
set consent to "not provided" -- because you're re-importing existing records their consent status will NOT change. If contacts already provided express or implied consent this will NOT be removed. The original consent status will be kept.