this is a quote from the manual
Changing Layers
You can change the instrument or sample of a layer slot.
Prerequisite
You have loaded an instrument or sample into one of the four layer slots of a drum pad.
Procedure
- Select the drum pad that you want to change.
- Select the layer slot that you want to change.
- Next to the layer slot, click the instrument icon.
- Select an instrument or sample from the layer selector to replace the current instrument or sample.
at step 3 it says click the instrument icon, I see no instrument icon?