Update multiple records by import and Export to Excel

Hi,

I have around 5000 contacts uploaded in Dynamics CRM for our company. 

Now I need to update around 500 of them and change their owners.

For 4000 of contacts, I need to update a value in a field.

If I do it record by record, this will take lots of time.

Is it possible to Export the entire contact list in Excel and I make all the changes in the required fields. Once all the changes are completed in Excel file, Import the changes back in CRM.

Also, If I import the data again and as 5000 records already exists in CRM, will they be updated or new duplicate records will be created.

I need to complete this task ASAP. Please reply with solution.

Regards

Deepak