If this relates to a Canon 1D mark ll you should have posted in the EOS section but either way I need to know if you mean the focus screen. If you do that part is owner changable because Canon sells a few different styles of them. There is a small tab in the middle of the frame that screen fits in which if pulled about 1/16 inch releases the focus screen. Invert the camera & let the screen drop into place then pull the clip lightly with a fingernail & very gently push the screen back into place, then release the tab.
Or do a YouTube search on how to change the screen in the 1 series bodies.
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