I don't think you can force the landlord to "replace" the A/C. You can ask him to repair it if it is not working properly though. A challenge that you may have is that he may feel the A/C is working just fine, while you think it is not. If you can't reach an agreement, I suggest putting your complaint in writing and sending it to him with receipt confirmation. You will want to have it documented that you gave him a chance to address the issue.
Jose Dias, REALTOR