Look in the options, I would highly recommend you familiarize yourself with the app a bit, do some tutorials and such before posting. Usually if you add something from the base components of the app, there are options you can use. They are right there at the very top and by seeing your code you should have recognized it had you looked there. No offense meant at all, just that it’s best to look around things, add something and then check your options at the top right and see what’s all there that you can do with it. Play a little bit, you’ll be glad you did.
Delete the additional cells, select the remaining cell and change Options>colspan to 2 (3, 4) or however many columns there are in your table. You may wish to center align the text in the header cell at that point as well.