Alter table with default value 'X'

How I can alter a field with default value ?
for example
I want alter c4 type char(11) DEFAULT 'x' NOT NULL;

I make
Alter table TABLE1
alter c4 type char(11), --OK change the size
add c7 CHAR (10) DEFAULT USER NOT NULL, -- OK new FIELD
add c8 CHAR (11) DEFAULT 'x' NOT NULL; -- OK NEW FIELD

but this not work

Alter table TABLE1
alter c4 type char(11) DEFAULT 'x' NOT NULL;

I receive the message on the DEFAULT word;
fmSQLEditor.Query:
Invalid token.Dynamic SQL Error.
SQL error code = -104.
Token unknown - line 2, char 24.
DEFAULT.

Thank you in advance