Map Values in 'nested' association

I have failed to find a good duplicate so:

assoc // Query[All, {"a" -> f}]

or

MapAt[f, assoc, {All, "a"}]

assoc = <|"key1" -> <|"a" -> 1, "b" -> 2|>, "key2" -> <|"a" -> 3, "b" -> 4|>|>;
assoc[[All, "a"]] = Map[f, Values[assoc[[All, "a"]]]];
assoc

<|"key1" -> <|"a" -> f[1], "b" -> 2|>, "key2" -> <|"a" -> f[3], "b" -> 4|>|>