This depends on how the stereo sample is done. If it is a "true" stereo sample
(which is a single sample with both sides of the stereo within the single
samples), then you will first have to split the sample in two, using the Split
soft button. You can then edit the Volume Adjust parameter on the MISC page. If
there are already separate left and right samples, then you can just edit the
Volume Adjust parameter on the sample you need.

Once you have split the two, you would then need to create two keymaps - one for
the left side samples and one for the right (which means you will need to split
all the samples that are used in one keymap), then assign the left side keymap
to Keymap1