Right click and Remove...
Use Calibre to manage your ebooks, on your computer as well as your Kindle.
If you are viewing your list of titles on your Kindle with Calibre, you can right click on any of them and choose to remove the title from your device.
If it is a book you have purchased on Amazon, there will always be a copy left on the Amazon server in your ebook archives. If it is a book you acquired elsewhere (like manybooks.net) and you delete it, it will be gone (gone gone), unless you have kept a copy either in your Calibre library, or in a folder on your computer.
If you don't have a backup, you can always go get it again... if you can remember the title?
God... Getting old is certainly not for sissies!