I just buy an apple Space Black Stainless Steel Case 40MM 4 days ago and I want to change 44MM, Can I change this?

If you bought it directly from Apple, the return period is 14 days.

If you purchased it from another retailer, ask them what their return policy is.

Call 1-800-MY-APPLE. Or go to the on-line Apple store for your country. Click on the icon to the far right that looks like a clip board, and select your "Orders". There should be a phone number listed there.

You return the current Apple Watch, then you buy the one you want.

Then you have 14 days from the date of purchase to return it.