can someone please help me do it

I'm assuming that you're asking this regarding RCT or RCT2. If so, you need a trainer to do this. I'm not sure which available trainers are able to do this since I don't use them personally. But, there are members that do use them often enough to help you out with your question.

what trainer do i use does anyone know because im having trouble using 8 cars

Here's a quick overview on how to do it using 8 Cars.

Open the trainer

Goto the "Rides" Menu

Click on "Edit Ride Stats"

Then follow the directions on the graphic I am attaching below.
Click to view attachment

****IMPORTANT NOTE****
Before using ANY trainer or performing any hack you should save your current game. Since the nature of a trainer is to edit the games temporary memory, the game may sometimes catch the trainer and you will get a "Error Trapper" message. Your game will then close without saving.

Hopefully that was a detailed enough little tutorial for you.