The only risk free is not to gamble at all. As per average, favorites do win most of the time, however, because of the odds, that would still make you unprofitable in the long run.
If you are good to go with just one shot of a bet, then bet on the underdog, but always remember that it could also lose.
Betting on the underdog will usually result to a loss. That's actually one way of not betting with minimum risk. If you are going to do that, just make sure you made enough research about that match and the potential winning rate for the underdog as well as understanding the gap from the odd.
Huge gaps usually an easy win for the favourite.
I can also say that betting with the favourite is actually profitable but with very less and minimal amount when you lost once, you'll have to get what you lost after several games betting with the favourites again.