Can a landlord change a lease without my knowing?

I was staying in a home with my girlfriend and my name and someone elses name was on the lease. When we broke up I wanted her to move but she wouldn't leave and the landlord took my name off the lease without me knowing and put hers on it, so I asked him is that legal to do without my notification and I should receive my security deposit back and she should be required to pay her own. Also the name that she's using on the lease is not hers and I alerted him of that being identity theft but all he cares about is rent money. My main concern is how do I get my deposit back and what other steps should I take?