Activity page redirector

Hi! I was wondering when a user log in, he/she is redirected  to "root/elgg/activity" page and similarly if selected "Activity" tab from menu too.

I want it to be customized to "root/elgg/activity/owner/$username", but I don't know which section of which file I should edit. Editing core matter isn't a problem to me.

Waiting for an early response.