You can often hold down the ALT key to move windows around, in order to find the OK/Cancel or whatever buttons when landed in a low resolution mode.
If however you want to do it all from the command line (and this will bring far greater flexibility to your use of Linux later on) then open up a shell, or switch to another virtual screen (perhaps CTRL-ALT-F2) and edit your XF86Config-4 file, eg.
# (root password)
cp XF86Config-4 XF86Config-4.old (make a backup)
vim /etc/X11/XF86Config-4 (or ed, pico or any editor you're comfortable with)
Down the bottom of the config file you should see the screen resolutions - just add the ones you want, save, and then re-start X.
Alternatively, on KDE there is sometimes a screen resolution changer named 'Screen Size and Rotate' in the system tray, bottom RHS, or under the K menu under System.