To add to what João Vitor said, if someone is 'used to' something it means that it is familiar to them and seems normal. Putting both parts together 'by then I will be used to it' means 'at that point this new thing or change will be familiar and feel normal to me'

By then means at a specific point in time. Example: You will go to Paris in the future and you're currently learning French. You can say: By then I will be speaking French. It means that when you go to Paris you'll be already speaking French.