Using External ID’s
Importing Data? Save yourself valuable time by using External IDs. All relational databases use a unique ID to tie data together. When importing different types of information (Accounts, Contacts, Opportunities, etc.), create a custom field on each type of record to relate one record to another.
Go to Object, Create new field, choose the Text field type.
Click Next and choose External ID
Once done, using the Data Load, choose Upsert
Choose your External ID
Finish the regular import process.