It is the one (unnecessarily) frustrating part of the game: there are three places labeled as "Terminal" but only one of the boards is the one you need to change. If the first one is not it, move one step forward (to the right) and repeat, and then again if the second did not work.
As it says on the mission description: "A maintenance check has revealed that one of the three boards
on the main computer has been altered."... I know <sigh>
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One more thing: Originally, the location of the one board that had to be exchanged was randomized each time you started that level (the whole New Washington level, not the mission), but since this port saves the complete state of the game it is likely that you will have the same one each time you reload, unless you start a whole new game.
The port that needed repaired was not randomized, ever. Unless this was a feature that was only in certain versions of the game. In 25 years of playing this game, it's been the same port every time without fail across numerous versions of the game.