How can this help?
When you are in a slumped position, your ability to breathe deeply is impacted.
Sitting up straight and tall can make it easier for you to take deep, slow breaths. This will allow your body to take the oxygen it needs from the air you breathe in, and will send 'all is well' messages to your danger monitoring senses.
Dropping your shoulders also helps you to feel more relaxed, as it soothes tense muscles.
You can find out more about why you would want to make it easier to breathe deeply from research like https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5455070
Find a short phrase which helps you remember this technique, like "head over heart, heart over pelvis" or "shoulders down, head high".
Then, when you're feeling low, remind yourself of this technique and give it a go.