Changing strings shouldn't void your warranty especially if you replace with the same kind of strings.
The first thing to do those is carefully lift that string out of the slot on the nut and move it to the side (you really don't even need to loosen it to do this). Brush the front face and the slot of the nut with something stiff but not to scratch (i.e. a toothpick or a long fingernail, if you have one). The idea is to clean out the slot and the front face of the fret around where the string rides. Often, during manufacturing there will be a tiny bit of flashing or what have you there and it will make the string buzz as it vibrates. Often just the handling of changing strings will knock something like that loose, which is why people often think they cured the problem by "replacing the strings." (Not that strings aren't sometimes bad.)
If that doesn't fix it then try the old trick of lifting the string out of the slot and putting a tiny piece of paper in the slot, with the string back over it. Tear off any excess paper (or it will buzz!) and see if your buzz goes away. If so, you know that slot in the nut is a little low. You can do a permanent fix with a tiny dab of superglue or (better) epoxy in the slot.
Most often, though, on an entry level uke the action is going to be high - which means if you've got a buzz and it isn't something simple like a bit of flashing on the nut or a bad string, the frets need to be leveled. Once the frets are sanded level you can then work the nut slots and, if necessary, the bridge down to get a really slick action.
This is where guys (and gals) like MGM and so on earn their keep - they set ukes up before they leave the store so you don't have to!
Since you're not terribly far away, if you can't get this worked out let me know and I can take a look at it maybe next weekend. Chances are, though, you can probably find somebody east of 360 that can help you more quickly - check out the Dallas Ukulele Headquarters gang and see if maybe somebody there can at least point you down the right path.
John