How to modify the legends produced by "Expressions" in the PlotLegends option

You could put y = in the functions

Plot[{y = x + 2, y = x - 2}, {x, -3, 3}, 
 PlotRange -> {{-3.1, 3.1}, {-4, 4}}, PlotLegends -> "Expressions"]

One way is:

f = {x + 2, x - 2};
Plot[f, {x, -3, 3}, PlotRange -> {{-3.1, 3.1}, {-4, 4}},  PlotLegends -> {Row[{"y", #}, "="] & /@ f}]