I just installed Linux redhat 8 and i forgot to put it in graphical mode can i change it from text mode to graphical mode

And how would i do it?

open /etc/inittab as root
change 3 to 5 at the beginnig

id:3:initdefault: to
id:5:initdefault:

Did you actually install the Graphical Mode software, or did you just accidentally choose "Text mode" in the boot up options? If you are just booting up in level 3, then the above advice is for you.

If you forgot to actually install X-Windows (XFree86) and KDE or Gnome, just boot up with the installation disk again and choose "Upgrade" instead of "Install". When you get to the packages part choose to select yourself what packages to upgrade and you should be able to install the GUI packages this time.

If you haven't done anything with the operating system yet and aren't worried about erasing any work you might have done, just go ahead and do a whole reinstallation. It is sometimes the easiest thing to do this early after the installation until you get some experience with Linux.