There is not a way to "mass complete" or complete all at once, a whole bunch of Activities. You will have to complete each Activity one by one.
To easily Complete the Activities that need to be completed, go to your Activity List (List menu -> Activities) and click on the "check mark" to the right of each Activity that you need to complete.
NOTE: The best practice is to Complete the Activity on the day or at the time, that it is done. Completing your Activities as you have done them, not only keeps you from falling behind and having to ask this question but it keeps your calendar clean and your Matrix Activity Report accurate.
*If you have a large amount of Activities that you would prefer to have deleted out of Matrix, you may contact your Success Manager for information on Professional Services for Matrix to perform Database Maintenance. We can preform a clean up service of your Activities where we will delete your un-needed Activities for a fee.