How to change PlotLabel after plot?

p1 /. Placed[a_, Above, b_] :> Placed[a, Below, b]

or (a bit more robust)

ReplacePart[p1, {-1, 2} -> Below]

General hint: Apply InputForm onto the graphics object p1 in order figure out what exactly to replace.

If Above appears only in relation to PlotLegends in p1:

p1 /. Above -> Below (* or *)
Block[{Above = Below}, p1]

enter image description here

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