When can I fix in on the 2 Year Fixed tariff?

I am at present on a 1 year fixed deal and it ends next month i am thinking of taking the option of a 2 year fixed deal but i wanted to know when i should change to a new deal , do i wait until the present deal ends or can i sign up before and if i do sign up before when will it come into force right away or at the end of my present deal
I have e-mailed OVO but no reply as yet so now looking for a new supply as time is passing and i wanted some information as i have over the last few days found energy companies are not to good at being clear . thanks in advance for any info