It depends on what you are importing. Individual things such as modules, pages, quizzes, etc. are added to their designated place (modules goes to modules and so forth). However, if you import a course it will replace old content.
Here are some guides for help:
Hello there, rayl...
I have been reviewing older questions here in the Canvas Community, and I stumbled upon your question. I wanted to check in with you because I noticed that we have not heard back from you since you first posted this question on February 16, 2018. Have you had an opportunity to review the response from @courtney_wagner and the links to Guides that she provided to you? Were those links helpful to you? Do you have any outstanding questions as it relates to your original question that we might be able to help you with? If you feel that Courtney has answered your question, please feel free to mark her feedback as "Correct". However, if you are still looking for some help from members of the Community, please let us know by posting a message below so that someone might be able to assist you. For now, I am going to mark your question as "Assumed Answered" because we've not heard back from you and because there hasn't been any new activity in this thread since the middle of February. However, that won't keep you or others from posting additional questions and/or comments below which are related to this topic. I hope that's alright with you, Leigh. Looking forward to hearing back from you soon.