How to copy all the test steps from one test case to another in zephyr scale?

There's a couple of different options depending on what you want to accomplish. You can export to excel and reupload (see MisterB's response).

Or you can clone the test case which will create a new, duplicate test case, then remove or add any new info (descriptions, titles, tags, etc.) on the cloned/duplicate test case. 

You can clone multiple test cases at once. There's also an option to bulk edit test cases.

Additionally, Zephyr Scale also has versioning, which allows you to create multiple versions of one test case.

If it's a test case or test steps that get reused often, you may want to check out the call to test feature.