i have a 2000 trailblazer 250 with a 13 tooth sprocket and was wondering if changing it back to 11 teeth would give me more top end.

 

A smaller front sprocket will reduce top end, would need to go to a 14 front sprocket!

 

Remember that changes on the front sprocket make large changes
From the factory you had: Front sprocket 11tooth, Rear 38tooth =3.45:1

If you change things the following happens.
Front sprocket 13tooth, Rear 38tooth =2.92:1

That is a big difference, You can quickly get to a point where the bike doen't have enough power to pull the gears. You can change the rear spocket for smaller increment changes.

Front sprocket 11tooth, Rear 36tooth =3.27:1

Its all about gear ratio.

Go up on gearing in the front "drive" sprocket, down on the rear "driven" and you would lower your gear ratio (Increasing top end speed IF you have the power to pull the gears(you will lose acceleration and torque)
Go down on gearing in the drive sprocket, up on the rear and you would raise your gear ratio (Increasing Low end torque and acceleration(you will lose top end speed)