Can't change settings after setting them wrong

Hello,
I get error warnigs from this plugin. They tell me to change settings - but I can't access settings any more.
1. message says: "Conflict warning: You'll need to turn off one of the past events expiry add-ons."
2. message says: "You do not have MANUALPAYMENTBUTTON keyword in the Instructions field. This means there will be no button and user will not be able to inform you that he/she made a payment, which in turn means that Manual Payment will be practically useless."

So, how do I reset the settings ?
And how to implement MANUALPAYMENTBUTTON ?