You would need to sit in the "Waiting Room" , im guessing you mean your FTC, so you would put your name with FTC 01 after it for example. "Fallen FTC 1"
Exit Arma,
Press Windows Key
Type Run
In the search bar put this %localappdata%
Find and open the Arma 3 Folder
Inside you should see a folder names "MPMissionCache".. Open that
Inside there should be a file named somthing along the lines of "RPUK.s1.prod.Altis"
Delete the .PBO File and...