DownValues

DownValues[f] gives a list of transformation rules corresponding to all downvalues (definitions for f[...]) defined for the symbol f.

Examples

Get function definitions as rules:

f[x_] := x^2;
f[0] = 1;
DownValues[f]
(* {HoldPattern[f[0]] :> 1, HoldPattern[f[x_]] :> x^2} *)

Modify definitions programmatically:

DownValues[g] = {HoldPattern[g[x_]] :> x + 1};
g[5]
(* 6 *)

Please visit the official Wolfram Language Reference for more details.