Make a variable for laps.
Every time they pass the start line increase laps by 1.
Then make an if laps = x then level = y | x is the lap you want it to change at, y is the level you want it to change to.
If you mean level as in the track, put a script on your background or track sprite that will change it like this.
if level = 1 set costume to costume1
if level = 2 set costume to costume2
Hope this helped. I didn't want to use scratch blocks D:
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