The 'X' button in the upper right is the button that closes the window.

Literal

Upper-right [at-にある] 'X-mark' [possessive-の] button [subject-が], window [object-を] erase button [copula-だよ].

バッテン ('X mark') is a colloquial term for the cross/X shape — likely from バツ ('wrong mark, X') + てん ('mark'). The sentence is a simple computer instruction. ウィンドウを消す ('to erase a window') uses 消す in its digital sense of closing/dismissing.