Instructions
Launch Windows Explorer. You can do so by clicking on the "Computer" shortcut in your Windows Start menu.
Navigate to the directory into which you installed "M.U.G.E.N" and double-click the data folder to open it.
Open the big folder and select the two files inside it, "readme.txt" and "system.def," by clicking and dragging with your mouse. Right-click and select "Copy."
Move back to the data folder, right-click anywhere inside that directory and select "Paste."
Click "Replace" to allow Windows to replace the existing files in the data directory with the one you copied.