Here's a snapshot of my race roster (populated from last week)---
use the blue arrows to replace any driver before the scheduled start of the race, after which those blue buttons will lock
use the green arrowed button on your garage driver to swap out any starter---once you click the green button, your starter buttons will turn yellow-click the driver you want to swap--you can do this as many times as you want before the end of stage 2---once you click the green swap button, a red X will appear above your garage driver--if you don't want to swap, click the x...if you've swapped a driver and change your mind, swap him back into your garage
your drivers from last race are now populated into your lineup for this week and you can play around with it now---I suggest you play around with the replaces and garage swaps now to make sure that function works for you :-)