One tenant wants to moveout

[Manoj N.]

I leased my single family home in Colorado to three ladies. They signed a joint and several lease on Oct 1st. One person wants to move out before the lease ends citing differences with others. She is looking for a replacement tenant. A few questions.

1. Can I refuse the replacement offer and insist that she stays till the end of lease (May 31)

2. In case 1 is not an option, Should I do background check/credit check of the replacement tenant ? What if none of her replacement meets my standard ?

3. How do I deal with her part of the security deposit ? They paid security deposit with a single check. 

Thanks