The easiest course of action to take when you want to sell property occupied by tenants that have a fixed-term lease is to simply wait until the lease has ended and they have moved out. You can't just go up to someone that has signed a rental agreement with you in good faith and tell them they have to leave. Now, if they have violated any terms of the lease then you can start eviction proceedings if they won't leave. But if they haven't violated their lease you do still have a few options.
First, you can sell the property to a new owner so long as the new owner accepts that there are tenants present and that the new owner will honor the terms of the lease. This will have no negative impact on the tenants, so there shouldn't be any issue with taking this approach. If this approach doesn't work for you then you also have the option of offering financial compensation to your tenant if they agree to allow you to break the lease. This option may cost you money, but if you are really eager to sell the property it is the option that is least likely to end up causing you headaches.