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 ?