How to make store bought whole multi grain bread softer?
What I've done in the past is mist some water on both sides of the bread slices, then put them in the microwave for 10-15 seconds (alternatively, an oven set at 300F/150C for 5-10 minutes will do it too).

Experiment with the amount of water (but keep it low) and cooking times. You should get much softer (and warm) bread slices out of it.