Down climbing is pretty self explanatory, as it’s simply climbing downwards. It is however worth pointing out that generally down climbing is much harder than climbing upwards. One of the reasons for this is because you are unable to recognise holds as climbing upwards.
If you have to down climb, take extra care. Many bouldering walls advise to down climb from each problem, rather than jumping off. This is a good idea as it reduces the impact on your joints. It is also a great way to double the amount of training for each problem climbed.