The weather suddenly got really cold, and I have phlegm. What should I do?

hello?
The weather suddenly got really cold.
So I can feel my respiratory system getting worse.
There is phlegm, especially in the back of my throat.
This is a picture of the inside of my neck, where you can't see it.
I feel like I have phlegm all the time.
It's not a lot to spit out.
It's even difficult to take medicine.
What should I do?