Customize the Minecraft World Options

According to this guide, you can adjust the difficulty by using the /difficulty command. If you play online, you need to edit the settings for your server in server.properties so that your changes remain after a restart. I've provided the direct instructions from the guide below so you can change the difficulty:

CHANGING THE DIFFICULTY VIA CHAT COMMAND

The easiest way to adjust the difficulty is to use the chat command /diffculty:

1. Open the chat via the “T” key.

2. Enter the command “/difficulty <desired difficulty="">”. For the hardest mode in Minecraft this would be “/difficulty hard”.</desired>

3. Now you confirm the input by pressing Enter.

Note that you need operator rights on the server to be able to change the difficulty level. This method is the easiest, but it has the disadvantage of having to repeat it every time you restart the server if you want to play on a certain difficulty level permanently.

CHANGING THE DIFFICULTY BY SERVER SETTING

A permanent change of the difficulty level needs to be done via the server settings. Proceed as follows:

1. Open the server and search the file server.properties.

2. Now search the line “difficulty” and change the numerical value according to the desired difficulty. For peaceful you enter 0, for easy 1, for normal 2 and for hard 3.

3. Save your changes, close the file and restart your Minecraft server.

You could also change your difficulty using mods, such as Roguelike or RLCraft. You can find more information about that here.