How to kill process in Ubuntu

I wanted to deploy my flask app in Ubuntu virtual server machine.
I was working with NGINX and GUNICORN. 
When I tried the command gunicorn flaskapp1:app  it showed me [ERROR] Connection in use ('', 8000)
Giving a google search, I figured out I must kill the running gunicorn process. I found the command is sudo kill pid 
How to find the pid? That was the question.
Using the command, ps -A (a capital A), all the process and associated process ids (pids) are shown. I identified the required process id and using the command, I killed the process. 
Thanks to all who helps with solutions.