How to redirect one page to another format

[jennypretty]

Hello,

I have many of web pages as:

domain.com/a_sites/
domain.com/b_sites/
domain.com/c_sites/

How do I use htaccess 301 formate to redirect to pages like:

domain.com/a_websites/
domain.com/b_websites/
domain.com/c_websites/

I only want to change from sites to websites.

Thanks very much!