Need help with update - Mailing list pgsql-novice

Hello.
My SQL is a little rusty,

With these tables (where id is unique)

Table Y:
 a  | id
----+---
 10 | 1
 20 | 2
 30 | 3

Table X:

 a | id
---+---
 1 | 1
 2 | 2
 3 | 3

Now I want to update each row in the Y table and set the a value to
the corresponding a value that is found in the X table, matched on the
id.

This means  that in the end, the Y table should contain the same
values as the X table in this case.

But how do I write a command to do that?