I have the following coordinates in meters for UTM 34S / Datum WGS-84:
Lat.: 311965; Long.: 8695153;
How do I convert these coordinates into decimal degrees or DMS?
You can do this in a dodgy way by using a form like this: https://www.engineeringtoolbox.com/utm-latitude-longitude-d_1370.html
This one looks a little more robust: http://franzpc.com/apps/coordinate-converter-utm-to-geographic-latitude-longitude.html
But as the comments suggest, to do it properly, you really need to know what datum you're on, and you've only provided info on your projection. You can get something that should be ok by assuming you're on the WGS84 datum, and your lat/lon will then be on that too. To do it right, you should use the right datum, though!