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You can change them around if your looking to emphasize a certian area more than another. I usually change around ab days so I hit the lower part of my abs first but other than that I try to stick to the workout laid out. Though there might be something to the order they give as my current work week to week seems to swap leg press and squats as my first two exercises every week... so i'd pretty much try to stay with what they have there unless your looking to really focus on one part of your muscle more than another...
Welcome to ft! No, you do not have to perform them exactly as they appear. Change is good, right? Anyway, what I do is usually do the first set of the first exercise. Then, I may skip and do the first set of the third exercise.
As Fireloard suggests, if you feel you need to work your upper chest (for example), then do the incline bench work first. Then work on the remainder of your chest program.