You can choose to show your heart rate either in Beats Per Minute (BPM) or percent of your maximum heart rate (% HR). Heart rate measured in BPM depends not only on the intensity of your training, but also on your age, sex, and fitness level. To be able to compare with everyone else, you should use your heart rate in % of your maximum heart rate. This is also what is used to calculate your training zones.
When you have selected heart rate as the focus on the recording screen (i.e., it will show as the big number in the lower part of the recording screen), simply tap the field showing your current heart rate. This will flip your heart rate between beats per minute (BPM) or as a percent of your maximum heart rate (% HR).