A joint lease is a rental agreement which has two or more tenants. When problems arise, it's usually because one tenant wishes to move out of the rental while the other one doesn't. It can be tricky to get your name removed from a joint lease. Most times, it's something that you'll agree to do through negotiation with the landlord and your co-tenants.
TL;DR (Too Long; Didn't Read)
You can't force a landlord to remove your name from the lease. Some landlords are willing to do so, however, so it doesn't hurt to ask. You'll also need to talk to your roommates.
Talk to Your Co-Tenants
When you signed the lease, you took on certain legal responsibilities. This includes a responsibility to pay your share of the rent. Because your co-tenants are also parties to the lease contract, you cannot remove yourself from it without leaving them holding the bag; that is, if you leave the apartment, they're stuck paying your share of the rent. So, start by asking the other tenants if they are willing to accept this additional responsibility. Sometimes, the landlord will voluntarily allow a new lease to be created with your name absent, but this requires the written permission of your co-tenants.
Negotiate with the Landlord
As a matter of law, you cannot force the landlord to take your name off the lease until the lease ends. A lease is a binding contract, and if you breach that contract, you're responsible, even if you have a good reason. You can, however, negotiate removal of your name from the lease with your landlord as long as your co-tenants agree to the revision to the lease. You may be able to pay a penalty in return for your name being removed, but this is entirely in the landlord's discretion. If you can negotiate a fee that's less than the remaining balance on your lease, it may be worth considering this option. Be sure to get an early-out payment in writing and make sure to get a receipt for the payment.
Find a Replacement Tenant
The landlord and other tenants may be more willing to negotiate if you can find a new roommate to replace you. This always requires the approval of your roommates and the landlord, and if they do not accept the change, there is little you can do. If you do find a replacement, your roommates and the replacement tenant will enter into a new lease agreement with the landlord with your name removed. At that point, your former roommates will not be able to come after you for any part of the rent that you agreed to pay when you signed the lease, because the lease is terminated and a new one is in place. This is preferable to an agreement that merely modifies the current lease to remove you and add the new tenant, but any agreement in writing is better than none at all.
If you find a replacement tenant and your roommates all agree to accept her but your landlord does not agree, be careful about going forward with the move anyway; if she does not replace you on the lease, you're still responsible for the lease payments if she doesn't make them or if the landlord kicks her out.
Get Legal Advice
Contact an attorney and discuss with him your legal options for removing your name from the lease. If you can prove illegal or dangerous behavior by one or more of your roommates, you may be able to get your name forcibly removed. In addition, if you can show that the apartment or house is uninhabitable and the landlord has ignored requests to fix the problem, you may also be allowed to terminate the lease early without penalty. This is commonly found with issues of unpaid utility bills that are the landlord's responsibility.