You can't switch between two different slots. That's not how it works. Any desired changes take place on the same slot.
Head to the tactics page and select your tactic. You should click here:
Then you set up whichever alternative tactic you want on the alternative (1)/alternative (2) areas. Following that you then set up a tactic rule to switch between initial/alternative tactics x minutes in or as a result of an event such a red card etc.
Qualquer coisa é só perguntar ;]