clear blue ovulation fertility monitor

Marcia • Currently Pregnant with baby No.2 EDD 11th March 2018 a little boy 💙 Got a beautiful little girl that was born 2015 💗
Hey ladies 
I'm sure one or two of you will know these answers,
My sister kindly gave me her clear blue fertiltiy monitor (not the advanced) 
She's gone away for a few weeks and I don't have instructions I've tried googling. But it's not making it any clearer 
So I don't think she rest it- how do I do this? 
when I turn it on it say M99 
I pressed the m button it goes from 99 to 1,2,3,4 
Help please lovelies I think it's too late in my month to start using it as yesterday was my peak fertile day so I'm guessing if this month isn't my month I start fresh with the new month? 
Thanks lovelies 😘😘