Use steam or a humidifier
A humidifier [2] modernizes an old but effective method of clearing sinus congestion. This equipment converts water into steam that you can inhale into your sinuses. Breathing in steam is a natural way to relieve sinus congestion. The steam helps break down mucus and helps it pass out more easily.
The steam you breathe in also moistens your mucus membrane and reduces irritation and inflammation in the blood vessels of your nose and sinuses.
If you do not have a humidifier, you can heat some water in a bowl and breathe the steam emanating from it. Cover your head with a towel while doing this to get as much steam in as you need. You may have to try this multiple times to get sufficient relief. Taking a hot shower [2] can also give the same effect because of the steam it fills your bathroom with.