altering the datatype of a column.

Dear my friends...

I want to change the datatype of a completed table
(has had already content).
But I don't know how.
Here is my try:
"
kv=# alter table customer alter column address set
type char(150);
ERROR: syntax error at or near "type" at character 47
kv=# alter table customer alter column address set
datatype char(150);
ERROR: syntax error at or near "datatype" at
character 47
kv=#
"

Please tell me how.

If I alter the datatype of the field, am I going to
loose the data inside the column?

Thank you very much in advance.

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