Old question, but if you haven't figured it out by now UTM is not a coordinate unit. UTM is a projected coordinate system (aka CRS, what I think you mean in your comment) whose units are meters. WGS84 is properly a datum, but also refers to a geographic coordinate system whose units are degrees. Projected is flat, geographic is 'round'. It just so happens that the UTM projection system also uses WGS84 as the datum. This makes things a bit easier to convert coordinates between, as since they're on the same datum no transformation is required.
The suggestions to reproject the data are because (at least in ArcGIS, probably in QGIS as well) if you field calculate an x or y coordinate it does so based on the current projection. So if your points are in UTM, and you reproject them to WGS84, and then field calculate the x and y coordinates, you will get the decimal lat/long coordinates you seek. This method would be easiest because it's all set up in the software already (and is essentially the proper way to do the conversion you're talking about). The alternative is to create your own manual formula to do the calculation and perform it on the coordinate attributes (which, btw, aren't actually tied to the geometry - changing those values would not affect the point locations or their projection in any way).