How can I redirect the login button to a different form in Flex theme?

[NaseerAhmed]

Urgent: 

I am using Flex theme in my store and I have installed an app called multivendor marketplace, however this app creates a very ugly misplaced button in the header section. To keep the overall aesthetic I am trying to replace the "Login (as customer)" button link and want to redirect it to the multivendor app registration form. 
Is there any way we can redirect the login button link to something else (in my case that multivendor registration form link)

Any help will be highly appreciated.