How do reset The enemy within?

[abc847782434]

Hi everyone!
Im wondering about the Enemy within. I chose the operation mode on the mission. But when I got to the area where I should enter it says: Free-to-play players are not allowed to participate in Operations. But when i chose the operation mode I got an instruction that told me that I could go back and change my choice to solo. Now my question is: How do i do that? Im a free to play player. Any help on this is greatly appriciated!