If you upload a CSV with new information for existing contacts or companies, any existing information will be overwritten by new values you've imported. New contact and company records will not be created for the records that already exist in HubSpot. HubSpot de-duplicates contacts by email address and de-duplicates companies by domain name and website URL. You can still view historical property values if a value for a property is overwritten.
For instructions on importing contacts and companies, check out this article.