beginner Table data type question

Hi, I just start using pgsql and I am new to SQL also.  I have read lots
of
documents on this and I cant seem to find the answer, perhaps you can
help me.

Say I have a table:

pgsql=> \d test_table

test_table
------------------------------
f1| CHAR (40)
f2| CHAR (40)

Somewhere down the line I need to change the data type on f2 to CHAR
(50) and add constrains like UNIQUE.  What is the best was to change
tables in this way?
Can I just make a new table the way I want and COPY the data out of the
old table and COPY it back in?  do I use CAST ?  Is there some ALTER
TABLE tablename MODIFY command I can use?

This question has been bugging me for awhile. Because I am new to SQL I
find that I make bad design choices about data types and such all the
time. So I need to know the best way to change them.

Richard