Redirecting www.mainsite.com/subsite/wp-admin to www.mainsite.com/wp-admin

i’m using groupblog plugin and hence multisite feature . there’s is one problem. Whenever an unregistered user goes directly to one of the subsites in my network , when he clicks login he’s redirected to https://www.mainsite.com/subsite/wp-login instead i want him to go to https://www.mainsite.com/wp-login. How can i do this ?