Switching SIM cards within the same phone?

I bought an unlocked iPhone 6 from the Apple store and they told me it can be used by any carrier. Little did I know I used a T Mobile SIM card when I activated it so it's using T Mobile. I have a brand new, never used Verizon SIM card and I was wondering if I could just switch them? And if I can, how do I exactly go about that? Just take out the T Mobile sim and replace it? Thanks.