sorry, cant help with changing a user to admin (I keep them seperate), or the why or not.
fortunately, with Linux, there is nearly always more than one way to achieve a desired outcome.
so, lets go for a walk......
launch Dolphin. right click on"/- Root" and from the mouse menu, choose properties, and on the tab, permissions. you should be presented with something like this.

- user dolphin.png (98.08 KiB) Viewed 1393 times
in the image, just below right of the mouse menu it lists ownership of the "/_ Root" partition as User: root and Group: root.
at the top of the tab, we have "Access Permissions" only the owner can change permissions.
note that the 3 next text info is greyed out . as you are not "root", you, as "user" can make no changes to the root partition, no new folders or files, as it should be.
lets do the same as on any of the "user" folders,(Videos?) and the "Permissions" tab will show ownership as "User: your_user_name" and "Group : your_user_name"
and in the top part of the tab, you will note that the formerly greyed out text boxes are now able to be edited.
as you are owner of "Videos", you can add, edit or delete files in this folder (directory).
check those partitions you cannot past a file too. they were created by root, so should be owned by root. you, as a user, do not have the rights to create a partition.
so, now exit/close dolphin.
so, to change to "root" without logging out, open a terminal and you should see this
- Code: Select all
[user@localhost /]$
the prompt "$" indicates normal user terminal.
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[user@localhost /]$ su -
Password:
[root@localhost ~]#
you are now in a root terminal, indicated by the # prompt. note the "-" after the "su" and a space
if you now type "dolphin" and press the "enter" key, dolphin will open with root permissions. you can now change the permissions of the problematic partitions and user and group by altering those boxes as shown above.
be aware that running graphical applications as root can cause accidental damage that may not show unit much later.do not change the permissions of the " /_Root" partition, or any files/folders below " / " in the directory tree. I thought it a good idea to mention this!
again, use with care.
close dolphin and exit the terminal. as a normal user, you should now be able to write files to, edit files in, and delete files in that partition.