Switch back settings so I can reach my files and download things again

Somehow I have personally switched off the ability to download things
from the internet. Every time I receive the pop-up screen
"Error opening directory. Permission denied" This is so for almost every file (Pictures, Documents etc.)
Also, when I try to add a file to an e-mail I get the same error message.
So I cant open my own files to read them, is my last discovery.

Could you please explain to me how I can go back to the optionmenu to change back my setting, so I can reach and add my files again and I can also download
things again?

Second question is where there is a general tour for all the "how to"
tips with Zorin Os. Do you have a manual?
Thank you in advance.

This sounds like a permissions issue. Something went wrong at some point, which changed the permissions or the ownership of your $HOME folder.
Let's assume both, just to be safe.

You can use the chown command to change ownership. Permissions can be changed with chmod.

You must use your $USER account name that you created when you installed Zorin OS. $USER is the generic stand in used for relaying how to type a command (since we do not know your account username on your computer). Always replace $USER from a generic command with your actual username.

sudo chown -Rv $USER:$USER /home/$USER/

find /home/$USER/ -type f -print0 | xargs -0 sudo chmod -Rv 644

find /home/$USER/ -type d -print0 | xargs -0 sudo chmod -Rv 755

This should change ownership of your home folder back to you and change all files and folders access back to you.

Swarfendor wrote a manual, here:

Sadly, this did not work, the username is typed incorrect.
The pre-installed OS version had a name I did not copy, because I did not
know this was important when I changed it in an easier name.

Is there a way I can trace the name that was given when pre-installed?
Is this logged somewhere?

Not that I know of, but if you changed your username, it should have changed system wide so shouldn't make a difference.
In terminal you can check your logged in user account with

whoami

Then, reference that with the one in root stored in environment

echo $USER

and see if they match.