Front deraileur not shifting to lowest gear. (New bike)

Yes you can, no you should not.

Since it is a new bike assembled by the shop, I strongly recommend you let them fix it. And it must be for free (more correct: you already paid for it, they did not delivery an adequate service).

You are inexperienced. There is a very slim chance the derailleur is defective (for example the chain guides are weirdly bent after a transport accident), a very large chance you do not notice it (if it is the case), almost certain the shop will void the warranty by your tinkering on it (they already demostrated their sloppiness in giving you a bicycle not properly assembled).

If you are ready to take this risk, the defect you describe may be due to the L(ow)-H(igh) screws position. You should be able to see them on the derailleur, hopefully they are labelled somehow. These screws are giving the boundaries to the derailleur movements. It is annoying because you have to release the cable, set the L limit so the derailleur sits a bit closer to the frame, hence guiding the chain to the lower cog.

Try to pose the same questions on notorious internet page for searching images videos and you will find plenty of walkthrough.

What can go wrong? worst case, the front derailleur will not move, or the L-screw will come off, leaving the derailleur with no bound to the lower movement, with your chain falling in between the cogs and the frame: are you ready to push the bike to the shop or to a close-by bicycle mechanics?

Ps: thinking twice about your bike assembly, test your brakes, try to simulate an emergency braking action. The bad thing is that the shop may have been sloppy also in that aspect, the good thing is that all these things have always to be sorted out during the first rides, even when the bicycle is set-up by a highly regarded professional.

Loosen the L limit screw on the front derailleur. No warranty will be voided for turning set screws. In this case all you need to do is turn that L screw about a ¼ counter clockwise. This maneuver should cause the derailleur to rest more to the inside, and the chain guided more inside as well. Makes for easier shifting. Turning either of these limit adjusting screws, L & H, more than 1 full turn should not be necessary, as adjustment should be done in 1/8 of a turn at a time (with attempts to shift on to the small ring in between ⅛ turns). Check how your shift is up from middle to large chainring. If this shift is crisp and normal, try turning the barrell adjuster--that would be in line with the shift cable, usually it's on the shifter right where the cable exits. It may possibly be further along the first stretch of cabling--clockwise a turn or two. This will decrease inner cable tension, allowing the front derailleur to more easily drop to the lowest chainwheel. You don't want to go too far with the adjuster and mess up the cable tension that is currently working on shifts up to larger chainwheel. If that happens you simply reverse the turns of the barrell adjuster. Counter clockwise rotation of the adjuster will tighten the inner cable tension. Try these easy end-user adjustments and you may be rewarded with proper shifting to all chain wheels.