Updating source install of zoneminder when it’s already installed is easy.

First, backup /etc/zm.conf (make install overwrites it)

git clone https://github.com/ZoneMinder/ZoneMinder.git
cd ZoneMinder/
git submodule init
git submodule update
cmake .
make
sudo make install

Put zm.conf back to /etc and done! Maybe check the sql directory for new db rollups.