Inflated tire partially slipped off rim, can I fix without deflating?
Short answer: no.

The only fix is to mostly deflate the tyre (so it's just barely inflated, enough to hold it in place but soft enough that you can manipulate the bead) and work the slack in the bead around the wheel so that you have an even distribution. If you're really lucky there will be a matching place elsewhere that the bead has stuck in the middle part of the rim, and when you push the slack around you'll find that and everything will pop into place. That's the ideal solution.

But this may also indicate that the tyre is slightly too large for your rim, in which case you're better off buying a different tyre. The way to find out is to fix it and see whether it happens again.

When you are fixing this it's tempting to just deflate, push the bead down, and re-inflate. The problem will occur again very quickly. The actual issue is that you have a circular bead that's slighter bigger than the circular bead seat on the rim. If you arrange them so that they're concentric (even all the way round) that will work, but it's not stable until you pump the tyre back up to push it up hard against the rim.

If you have a weird bike with odd-sized wheels it's vaguely possible that this is actually the wrong size tyre altogether. But that's unlikely. More likely you have a cheap bike with slightly undersize wheels, or a damaged rim, and a generously built (cheap) tyre.

It's worth buying a bike pump, not just for times like this, but so you can keep your tyres pumped up which will make riding easier. And it's a step on the way to owning a puncture repair kit (which will save you money as well as time).