You can use Guitar Pick or thumbnail to open it.

Take out this Covering Strip.

Release the Screen flex cable.

Be careful, mind the Screen flex.

Remove the worn Navigation Button.

There are two protective film, green and transparent.

Remove the transparent film.

Now the central part is nothing left.

Put it on the screen.

Make it match to the screen.

Make the sticker matched on the sides.

Tear off the protective film.

There is another green film, peel it off.

Be careful don't touch the sticker on the middle.

Mind the flex.

Hold it.

Install a new Navigation Button.

Make the screen match to frame.

Connect Screen flex cable and put back covering strip.

Put back the Battery.

Source: Youtube