You need to run the program UPDFILE.  The default company is the one in the Change Company list with empty brackets after the name.  if you have nothing listed on the Change Company list with empty brackets, then you are already in it.  If you do have such a company listed and can't change to it, you need to go to PS-C and add an access level for your login ID so you can go there.
 
Once you are in the Default company, go to UT-A, program UPDFILE.  Click OK on the messages and in this particular case you really don't have to get other users out because you will be creating a new file, not updating an existing one.  Scroll down the list of files to ISARCHG, click so it has a checkmark, then click Process.