how do I change the F3 and F4 to copy and paste? Using the Magic Keyboard, how do I change the F3 and F4 back to copy and paste?  The Apple Magic Keyboard with High Sierra launches mission control or something else.
Open System Preferences->Keyboard. Disable F3 first:



Then add an application keyboard shortcut:



Make sure that the text matches the menu item exactly ("Copy" in your case).

PS: Repeat for Paste/F4