The cleanest and most efficient way to evaluate polynomials is
defining the appropriate PolyAlgebraHom.
However there are some handy shortcuts: subst and eval.
Example
/**/ use R ::= QQ[x,y,z];
/**/ f := x+y+z; --> let x=2 and y=1 in f
/**/ phi := PolyAlgebraHom(R, R, [2,1,z]); phi(f);
z +3
/**/ eval(f, [2,1]);
z +3
/**/ subst(f, [[x,2],[y,1]]);
z +3