How to change column datatype from character to numeric in PostgreSQL 8.4

You can try using USING:

The optional USING clause specifies how to compute the new column value from the old; if omitted, the default conversion is the same as an assignment cast from old data type to new. A USING clause must be provided if there is no implicit or assignment cast from old to new type.

So this might work (depending on your data):

alter table presales alter column code type numeric(10,0) using code::numeric;
-- Or if you prefer standard casting...
alter table presales alter column code type numeric(10,0) using cast(code as numeric);

This will fail if you have anything in code that cannot be cast to numeric; if the USING fails, you’ll have to clean up the non-numeric data by hand before changing the column type.