Ideally you would get these contacts into a grid like with an Advanced Find and then use the Export to Excel option and checking to make the data available for re-import. From there you can edit in Excel and then use the Data Import (Settings -> Data Management -> Imports) feature to update the existing records.
Since you already have a spreadsheet with the updated data you can copy the updated data to the sheet you exported from CRM or if you are good with Excel perhaps use Vlookups to read data from one sheet and populate the other.