Trying to sell property but tenant doesn't want to give keys

[Ayodeji Kuponiyi]

I recently took over 2 properties that were deeded to me by my dad and plan to sell them to purchase an apartment building. One of the properties is a duplex with one unit vacant, the other occupied. I have a lock box on the property for prospects to see the vacant unit. I just found out the tenant changed their keys for their unit and I need to show their unit. The tenant offers to be accommodating which is great but what if I need to show their unit when they're not home? I've already requested a copy of their key but what else can I do? Thanks in advance