When using the import feature (Tools | Import) of the CRM application (as opposed to the Data Migration Manager) it is possible to update existing records rather than adding new records.
To do this, ensure that the first column in the import file is named the same as the import record type (e.g. if importing accounts then the first column must be named account) and that the data for this column contains the GUID of existing records.
To obtain the GUID for existing records, use the Export to Excel with the Dynamic Worksheet option (or the static worksheet option if you have Update Rollup 9 or later installed). The Excel file will contain the GUID in an hidden column. Select the whole worksheet and then right-click and choose unhide.
The Import Wizard will detect this column and offer an option “Enrich data by updating existing records rather than creating new records”. Note that this option only seems to appear if the Data map is Automatic. The Data map will only be Automatic if the column names in the import file are exactly the same (case sensitive) as the display names on the record form.