apt repository for WeeWX

Tell the system to trust weewx.com

sudo apt-get -y install wget gnupg
wget -qO - https://weewx.com/keys.html | sudo gpg --dearmor --output /etc/apt/trusted.gpg.d/weewx.gpg

Tell apt where to find the WeeWX repository

For Debian10 and later (WeeWX5 and Python3):
echo "deb [arch=all] https://weewx.com/apt/python3 buster main" | sudo tee /etc/apt/sources.list.d/weewx.list
For Debian9 and earlier (WeeWX4 and Python2):
echo "deb [arch=all] https://weewx.com/apt/python2 squeeze main" | sudo tee /etc/apt/sources.list.d/weewx.list

Install and/or Upgrade WeeWX

sudo apt-get update
sudo apt-get install weewx