You can use the following rule to redirect from https to http :
RewriteEngine On
RewriteCond %{HTTPS} ^on$
RewriteRule ^(.*)$ http://example.com/$1 [NC,L,R]
Explanation :
RewriteCond %{HTTPS} ^on$
Checks if the HTTPS is on (Request is made using https)
Then
RewriteRule ^(.*)$ http://example.com/$1 [NC,L,R]
Redirect any request (https://example.com/foo) to http://example.com/foo .
$1 is part of the regex in RewriteRule pattern, it contains whatever value was captured in (.+) , in this case ,it captures the full request_uri everything after the domain name.
[NC,L,R] are the flags, NC makes the uri case senstive, you can use both uppercase or lowercase letters in the request.
L flag tells the server to stop proccessing other rules if the currunt rule has matched, it is important to use the L flag to avoid rule confliction when you have more then on rules in a block.
R flag is used to make an external redirection.