Navigate to the following directory:

cd /usr/share/plasma/look-and-feel/org.kde.breeze.desktop/contents/components

Then edit Clock.qml:

sudo nano Clock.qml

Find the following line: text: Qt.formatTime(timeSource.data[“Local”][“DateTime”])

And edit this line to the following:

text: Qt.formatTime(timeSource.data["Local"]["DateTime"], "hh:mm:ss")

Remove :ss to just display hour and minute. Finally reboot.

Sourced from askubuntu.com - Change time format on lock screen to 24h