And so it begins, I have been expecting the annual flush of Christmas Junipers. If you have been to Harry's site (Bonsai4Me) and read [url=https://www.bonsai4me.com/SpeciesGuide/Juniperus.html]this,[/url] then you are already aware that you are facing an uphill struggle that may not end well. Harry writes:
"Never try to grow Junipers indoors, though they may seem to tolerate indoor cultivation at first, poor humidity, lack of light and dormancy will eventually kill them. Do not trust a vendor, seller, book or website that claims Junipers can be grown successfully indoors! Dead Junipers can continue to display normal foliage colour for weeks or even months after they have effectively died."
The fact that your tree is already browning is cause for concern, it is already in distress, not surprising considering it came in a box. I never really know what to say in situations like this. I can't suggest putting it outside now, and keeping it inside is not appropriate either. You, generally speaking, should never be forced to make such choice.
To place it on your balcony after coming, presumably, from a greenhouse somewhere is a problem. It is not likely to be acclimated to the extremes of temperature that a tree that has been outside all summer and fall could withstand. Since you live in an apartment you are at further disadvantage in that you have no cool place (sheltered porch or spare room) to attempt a transition. Leaving it inside is not much better. The hot, dry air and low light is not conducive to the health of any tree but a Juniper is one of the least likely species to do well inside, sorry. Some do try, but without supplemental lighting and an artificially induced dormancy the chances of success are slim.
If you are determined to give this a try I do have a few thoughts. Read the sticky thread at the top of this forum for tips on watering, this will be your first hurdle. Despite what the box says bonsai is not a particularly easy endeavor. Even watering takes some time to master and the horrible soil that your tree is in only makes things worse. Give the tree as much light as you are able, most indoor growers use artificial lighting to replace/supplement any natural lighting that may be present. Make a humidity tray, this is a shallow tray with no holes that holds gravel and water. It is intended to raise the humidity in the immediate vicinity of the tree, misting would not hurt either. I would also suggest the coolest location available but in your situation it does not sound like that is possible.
Sorry to be so pessimistic, and I know you are not to blame, but it is better to look at this situation critically from the outset than to raise false expectations. Sometimes Christmas gifts are returned, is that an option in this case? I realize that you may not wish to offend the giver, but there are other species that at least would have a fighting chance in your apartment. Know that all bonsai growers kill a few, sometimes a lot of, trees. If this one should perish it does not have to be the end of bonsai for you.
BTW, welcome to the site and to bonsai, good luck.