Did you figure it out "temporarily" or "permanently".
The temporarily way is to change it at the bottom of the treads page in the Display Options box by selecting an item from the drop down list box titled "From the" then clicking on the "Show Threads" buttons.
The "Permanent" way is to go to your User CP hotlink at the top of the page. Then click on the "Edit Options" link on the left hand side of the page. The 3rd MAJOR box down is titled "Thread Display Options" an there is the "Default Thread Age Cut Off" selection. Select the number of days you want the threads to display from this list.