altering a column to to make it generated

Can I alter a table column to now make it generated? I tried this unsuccessfully:

create temp table tmp1 as select 15::numeric(6,1) as distance,'24:30'::interval,0::numeric(7,3) as avgspd;

alter table tmp1 alter column avgspd type numeric(7,3) generated always as ((3600.*distance)/EXTRACT (EPOCH FROM rdrtime::interval)) STORED;
ERROR:  syntax error at or near "generated"
LINE 1: ... table tmp1 alter column avgspd type numeric(7,3) generated ...

I think that at least I have the right bracketing this time! :-)                                                             ^