This baffled me for days, but someone else pointed the way out.
There are two ways to go through the DTP menu when you have opened up your book title. If you use the menu on the left of your screen, you get the result that you (and I) experienced: once you upload, you can't upload a second time.
But there is also a menu horizontally across the top of your screen. If you go to Upload there, the Browse button will magically look healthy and frisky again, and you will be able to find your file and upload it.
I'm new. I had trouble with the browse button on my first upload, but clicked it several times in the area and it finally just worked. I'm ready to upload a second ebook but it doesn't work at all now. I've restarted both the draft and my computer, I've logged out and in, it has been several days and I'm still stuck. Is Amazon aware of this problem? I'm using firefox. I don't see other toolbars at the top or sides to try. Help!
Egads! I'm having this problem again too! When I click on the Upload New Content button, I get a window to put the cursor and a browse button. Neither work! I'm clicked out and in, logged on and off, cursed, shouted, nothing works. Amazon, all I want to do is upload a new and improved version of my book so your customers will have a better reading experience. Can you help me do this?