How to use htaccess to change to ?

If you only want directory become, use:RewriteRule ^catalog/$ / [L]

If you want all directories become, use:
RewriteEngine On
RewriteBase /

RewriteRule ^([^/]+)$ /catalog/$1 [L]
RewriteRule ^$ /catalog/ [L]

Note:If you have other subfolders in there (in that catalog dir, you’ll need some additional rule for that also. This will rewrite your files from catalog/ subfolder to your main url.