@Guillaume Slow, measured breathing (like box breathing). Keep doing it till your body responds - it will. You can start by yawning or coughing. The coughing helps to jolt your body to reset the breathing. The yawn can help create a cascading relaxation response.
Also do some rhythmic movement, like 5-10 mins moderate walking/stretching. It helps calm the polyvagal system. Look for friendly faces. Seeing signals of safety (e.g., smiles), helps elicit l the social engagement system.