You're able remove or edit most parts of any of our templates. However, if you already have one content block selected in your email, you won't be able to select another block to make edits to it until you save or exit the one that you're currently working on.
If you see Cancel or Save as an option on the bottom right, that means that you currently have a block selected. You can click either Cancel to de-select it, or click Save to save the changes you've made. Then you'll be able to select another block.