All you need to do is add a line like:
RewriteRule ^aboutus$ /about-us [R=301,L]
In to your .htaccess file. The old url should go between the ^ and $ and then new url after the slash.
All you need to do is add a line like:
RewriteRule ^aboutus$ /about-us [R=301,L]
In to your .htaccess file. The old url should go between the ^ and $ and then new url after the slash.