Must I go to the program files and enter the text there in a file? As I have a lot of users I can not risk to mess up my wiki.
kind of. but it's easy. Don't modify anything, just drop your _header.txt file in a folder and everything will be ok. If you put the file in the wrong folder, nothing will happen, neither good nor bad. It is always a good thing to have backups and to know how to restore; just in case.
First prepare your _header.txt text file, using a simple notepad program (not a word processor)
put your wiki text in that file.
Then, use a filemanager/ftp-program/whatever and open the folder where DokuWiki is installed.
From there, enter the
data[/m] folder, then enter the [m]pages folder.
In this folder, copy your _header.txt file.
Now, open your wiki in your browser. You should see your header. If not, purge doku's cache : the simplest is you go to admin, configuration settings and you save your settings without changing anything.