My current main domain for my hosting account is going to expire. How do i change it? 

It's not clear what you're asking. The domain name registration is expiring, your hosting account with LP is expiring, or both? If the domain name registration is expiring, do you want to renew it or let it lapse and pick up with another name for your site (which needs to be registered)? Who is the domain name registered with? What do you want to renew, and what do you want to change? Please clarify what you're looking to do.

If you want to change the domain name (and it is expiring soon and you don't want to renew the current one), yes that's possible. Letting a domain name expire, registering a new one, and changing over your account's primary domain name are three separate things, so clearly explain to support what it is you're trying to do.