Wife has a brand new Trek Skye S. It has a 24 speed drivetrain with the Shimano Altus EF51, 8 speed shifters.
The problem is that the (right hand side) thumb lever has no return spring action. Once you push the lever forward with your thumb to downshift it does not return. Instead it just remains in the forward position. You have to manually pull the lever back into the original position in order to continue downshifting or if you want to upshift with the index finger lever on top.

How do I adjust this thing?

Thanks!