Shimano Altus 9x3 speed shifter leaver is soft
If there are any problems with a bike after a week you should take it back to the store where you bought it.

If that's not possible or you bought online etc., it sounds like there is no tension on the shift cable, or possibly the shifter is broken.

The cable may have become detached from the rear derailleur, or pulled through the pinch bolt. A section of housing may have come out of a frame stop, or the cable may have broken (unlikely on a brand new bike). Possibly the derailleur is jammed on a low gear (large sprocket) so the spring is not pulling the cable.

Inspect the cable run from the shifter to the derailleur, making sure the housing is in the frame stops and is attached to the derailleur.

If that does not reveal a problem, detach the cable from the derailleur by undoing the pinch bolt.

Inspect the derailleur, can you force it to change gears by pushing the parallelogram while manually turning the cranks? Be very careful to not get fingers stuck in the chain while doing this!

Grab the cable (pliers may be needed) and put tension on it. Work the shifter. If the shifter is not working and releasing and pulling cable as you shift up and down then the shifter may be broken.