No, they do not mean the same thing. If your talking about a meeting then saying "maybe I could push it" expresses considering moving the time of the meeting up by 30 mins or an hour or even a different day. For example, maybe I could push the meeting up for an hour later or maybe I could push the meeting up from 2pm to 2:30pm. This sometimes occurs when something comes up and the meeting can not proceed as planned if your the one hosting the meeting. When you say "maybe I could cancel it"will express cancelling the meeting completely. To cancel something will mean that is the end of it. It's not used lightly or delicately. People will take these words to mean that the meeting is not happening at all. This is especially if it is not rescheduled. However, if it is important rescheduling is anticipated by some people. I hope this helps you.