When viewing a product in your own store, you can edit the product simply by starting to make changes in the page. When that happens the product enters Editing mode. This is shown by the addition of a notice at the top of the page:
Each change you make is automatically saved in the database; there's no need to click on any "Save" button. However, these changes aren't visible to users that look at the product in the market until you click on Accept changes.
If the product was Active before you started editing the product then the previous version continues to be shown in the market. Users are not aware that the product is being edited.
Click Cancel changes to discard the changes that you've made to the product and return to the last saved version.