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Enthalpy is a state function, so a reaction's enthalpy depends only on the start and end points and not the route taken. Any set of steps that adds up to the target equation has enthalpies that add up to the target enthalpy. Reversing a step simply flips its sign, which is why the reverse route is the negative of the forward total.
Hess's law
dH(overall) = dH1 + dH2 + dH3
Reversed route
dH(reverse) = -dH(overall)
Enter it with its sign flipped, or with its enthalpy multiplied by the factor you scaled the equation by, before summing.
Because enthalpy is a state function. The energy difference between two states cannot depend on the path, or you could build a perpetual energy loop.