There are two ways you can adjust the game difficulty.
First, you can do it from the main menu by clicking the difficulty option shown on the screen below. (it's highlighted and says "Difficulty: Normal")
That will bring you to this menu. There are presets at the top which you can select, and then adjust according to your preferred level of difficulty.
You can also access the difficulty options by pressing the pause button to bring up the in-game menu, and selecting difficulty from there (it's highlighted and says "Difficulty: Normal" in the following screenshot).
Which then gives you these options, the same as if you had picked from the main menu.
You should not need to reboot the game for these settings to take effect.
You can place your cursor over the various options, or activate the arrows, to make the tooltip on the right tell you what each option does for a completely custom experience!