It depends on what you need to do. You can use replace since you want to replace the value:
select replace(email, '.com', '.org')
from yourtable
Then to UPDATE your table with the new ending, then you would use:
update yourtable
set email = replace(email, '.com', '.org')
You can also expand on this by checking the last 4 characters of the email value:
update yourtable
set email = replace(email, '.com', '.org')
where right(email, 4) = '.com'
However, the issue with replace() is that .com can be will in other locations in the email not just the last one. So you might want to use substring() the following way:
update yourtable
set email = substring(email, 1, len(email) -4)+'.org'
where right(email, 4) = '.com';
See SQL Fiddle with Demo
Using substring() will return the start of the email value, without the final .com and then you concatenate the .org to the end. This prevents the replacement of .com elsewhere in the string.
Alternatively you could use stuff(), which allows you to do both deleting and inserting at the same time:
update yourtable
set email = stuff(email, len(email) - 3, 4, '.org')
where right(email, 4) = '.com';
This will delete 4 characters at the position of the third character before the last one (which is the starting position of the final .com) and insert .org instead.
See SQL Fiddle with Demo for this method as well.