On the Humax Freesat remote control the second button up from the bottom on the left hand side is marked Schedule. All you need to do is to press that and it will bring up your recording schedule. Scroll up or down to the recording you want to stop and press OPT+ as lotvic says. If you want to delete more than one recording then also do as lotvic says and put a tick in each box then press OPT+
lotvic I'm not sure how I originally found it - certainly not by looking at the instruction book - but with my box if I just click on Guide the sound goes while the programmes are being updated and it takes several minutes before it comes back, but if I click on Schedule and then the Red button I get the programme guide up and running with sound and it seems easier to glide through the day's programmes.