[Vanilla] How to change part of the UI?

[gqqnbig]

In AddOverviewOurRelationship() in Base\Assets\UI\DiplomacyActionView.lua, I want to add a tooltip.

        if (GameInfo.DiplomaticStates[iState].StateType == "DIPLO_STATE_DENOUNCED") then
 ...
        elseif (GameInfo.DiplomaticStates[iState].StateType == "DIPLO_STATE_DECLARED_FRIEND") then
...
        else
            overviewOurRelationshipInst.RelationshipText:SetToolTipString("haha"); -- my line
        end

If I edit the original file, I only need to revise nil to "haha", a few characters.

Now, If I make it a mod, what should I do?