You haven't specified much information, but it sounds like you are trying to do this in batch fashion, outside of any GIS type applications.
You can use GDAL to do raster reprojection.
You didn't mention what OS you are running, so I can't provide specific help for the setup of GDAL and such, but it's generally pretty easy for Windows and Linux.
After you have it installed you can use
gdalwarp to convert from UTM to WGS84:
gdalwarp -t_srs '+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs' <input_name>.tif <output_name>.tif
If the file is in WGS84 already, but its tagged as UTM then you can do the below (this just assigns a different system, no reprojection):
gdal_translate -a_srs '+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs' <input_name>.tif <output_name>.tif