1440 minutes = 24 hours.
How can i change the "Users active in past 1440 minutes, to say 24 Hours instead?
You *really* should not abuse that system for that purpose.
Aside from completely screwing up who is listed as online (because anyone who visits the site will still be marked as online for the next 23 hours and 40 minutes, give or take), the performance hit is quite sizeable.
Instead you should use the Users Online Today mod.