Undergrad Why Replace u with Positive s in Integral Manipulation?

[Woolyabyss]

I was looking over my lectures notes and became stumped when the lecturer began manipulating an integral using a dummy variable. I've attached the integral below as a JPEG. I can follow the substitution up until the very end but am having trouble understanding why he replaced u with a positive s and not a negative s at the end.
Any help would be appreciated.

 

[mathman]

u and s are dummy variables for the integral. It doesn't matter what letter is used. I suppose he used s at the end, since the dummy at the beginning was s. Once the integral is evaluated neither s or u will appear.