Trek Skye S - Cannot downshft!
 

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!

Brand new Trek says "Take it back to the seller to me."

It's probably something real simple like dried up grease inside the shifter or something binding up in the shifter. If the shifter itself is defective however, do you want to muddy up the warranty issue by trying shade tree remedies.

+1 Brand new says let the dealer take care of. It's possible that it's a simple problem like a small chip jamming the spring action, or it might be an unrepairable lever defect. You have nothing to gain by tampering so let the dealer deal with it.

Definitely , if anything like the LBS, a Trek dealer , here.. the shop will do the adjustment/trouble-shooting /education, 1st thing..