You need to specify which coordinate system is being used. It is probabably UTM, based on the tag in your question. If so, you will need to figure out which UTM zone the location is in.
You can lookup which UTM zone the location is is using a variety of resources including online references such as this one.
Once you have this information, there are many online converters you can use. Just Google 'UTM coordinate converter'
(If the coordinate system is NOT UTM, then you will need to find a coordinate converter for which ever system it is. Or use a GIS to create a feature with those coordinates in their native projected coordinate system, project the feature to a geographic coordinate system and then check it's new coordinates.)