Merge duplicate company records

We’re working to clean up and update our company records, but are struggling to merge records and data for duplicate companies. I want to bulk import new data to add to existing company records, but can’t figure out a way to do that without creating new company records. And then there’s not a clear way to dedup those company records. My account manager said there’s a way to do this via spreadsheets but haven’t gotten an answer. Anyone else tackled this problem??