Select the contacts you want to update.
Update the values in the country column in the exported list to use a supported ISO country code or country name.
Remove unrelated columns from the file - leave only email and country.
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.