To remove entries one at a time screenshot 

First begin typing the address. Select the outdated address.

Apple screenshot 2

Next, click on the blue oval around the address.  A menu will pop up.  You can then click "Remove from Previous Recipients List" and the address will be removed and not be used for autocompletion in the future.  You can then backspace over this address and begin addressing the message to the proper address.

To remove multiple entries at once screenshot #3

With open, select Window->Previous Recipients. screenshot #4 

This window will pop up, you will then be able to select the addresses and click "Remove From List".