Question : htaccess Partial Query String Rewrite

I have a client that changed his store code which changes one parameter in the query string.

See Store_Code=ABC in this link.
mysite.com/merchant.php?Store_Code=ABC&Screen=CTGY&Category_Code=WS-S

Using .htaccess modrewrite,
how do I rewrite this so it becomes Store_Code=XYZ
mysite.com/merchant.php?Store_Code=XYX&Screen=CTGY&Category_Code=WS-S

OR remove it so it becomes
mysite.com/merchant.php?Screen=CTGY&Category_Code=WS-S

Example;
RewriteCond %{QUERY_STRING} ^Store_Code=ABC$
RewriteRule what goes here?

Answer : htaccess Partial Query String Rewrite

This should look like this:

RewriteCond %{QUERY_STRING} ^Store_Code=ABC(.*)$
RewriteRule ^/merchant.php$ /merchant.php?Store_Code=XYZ%1  [R,QSA]

Regards,
Arty