The 60-days timeframe is a strict rule. You can not extend that. If you are unable to meet that deadline, then you can decline an invitation. You will be put back into the Express Entry pool of candidates and considered in future rounds (if you are still eligible). Please note that declining an invitation will not affect whether or not you are invited to apply later, as long as your score is high enough to get invited. However, if you don't decline an invitation and you don't apply within 60 days, the invitation will expire, and your profile will be removed from the pool. You will have to start over again.
