In Ableton 10 and higher, you can simply place the 'Utility' plugin on your effects chain and select the 'mono' button.

You can switch from stereo to mono in the control room. Activate the control room. and click the stereo tab next to the monitor. If the stereo tab is blank, you can check that stereo and mono are active in the down mix preset.

Turn the stereo separation knob all the way to mono in the mixer.

You can Use the Utility plugin. Open the Inspector menu, click the Audio FX slot, hover over Utility, and select Gain. In the plugin's interface, toggle mono on and off. Or use the Mastering Assistant: turn the knob all the way to the left to create a mono mix.

To sum to mono in Universal Audio's Luna, you can use the Monitor strip to switch to mono and send a mono mix to the Monitor outputs.

On the master fader, insert the Air Stereo Width plugin and dial width to 0%. You can also use the Hitshaper's free Mono plugin. This plugin can sum to mono and route the full signal to either a left or right speaker output.

You can use the mono button in your master track options. The mono button allows you to quickly switch between mono and stereo.

Place a stock M-CLASS stereo imager rack extension on your master bus, dial to mono and then simply toggle the bypass switch to go back and forth between mono and stereo. There are also many other rack extensions / plugins such as the, free to download, Anansi M/S rack unit which has a simple mono / stereo toggle switch.

Intuitively, you can simply toggle between stereo and mono on the master bus using the stereo / mono icon.