You can do this in Process | Find/Change.
1. Select all the notes you want to fix.
2. In Find/Change screen 1, Click ALL and OK.
3. On the second screen, click All and then in the Duration box, put the desired number of ticks in both the Min and Max boxes and click OK.
For example, if you are using 480 clicks per quarter and you want to set everything to sixteenth notes, put 120 in both boxes. If you are using 120 clicks per quarter, use 30 in both boxes for sixteenth notes. It sounds long but really just takes a few seconds.