Good morning, @arjanakadiu59 Welcome to the Canvas Community. Thank you for posting your question. You really don't "open" the Rich Content Editor (RCE). The RCE is available to you whenever you edit content in your course...be it a course page, an assignment, a discussion topic, a quiz, etc. Here is a great video tutorial on what the RCE is:
Also, if your school has enabled the New RCE (which will be available for all Canvas customers this Summer), you can check out this video tutorial:
There are also written Guides for each of these:
I hope this information will be of help to you, Arjana. Please let Community members know if you have any questions about this...thanks!