Since this action can't be reversed with Edit > Undo (or Control / Command + Z), you should first go to File > Save As and rename the file so you'll have a copy of the script with revision marks and a copy without revision marks.
Go to Edit > Select All.
Go to Production > Clear Revised.
This will clear all revision marks and revision colors. No text will be deleted.
NOTE: If you still see revision marks and revision colors after you choose Clear Revised, you may need to refresh your screen. Simply scroll the page up and down, which will cause the program to redraw the script. When it does, the script will be drawn in without the marks and colors.