How do I decrease the screen brightness in f.lux?

You can increase brightness with Alt+PgUp and decrease brightness with Alt+PgDn.

As the OP's answer specifies, you can increase and decrease brightness with Alt+PgUp and Alt+PgDn.

However, on Windows, these hotkeys were disabled by default for me. You can activate these by clicking on the three-horizontal-bar menu and selecting "Options and Smart Lighting...". Here, tick the box to enable hotkeys.