Update to crm 2015 on online organizations

Hi,

I would like to ask if the update to crm 2015 version is obligatory. Is there a way to  postpone because now I have  crm 2013 online organization which is not a trial.

Also if updates are already done, is it possible to roll back to previous version? I had an crm 2013 online organization, the new update occurred and now there are a lot of problems and errors. 

Thanks is advance.

Best regards