In Dropbox, select the Share button at the top right, then tap Open In... and select Quickoffice. There you can edit and save the file. Saving it back to Dropbox works the way I showed it in my previous post, and it will create a new file. So you'll have both versions there. If you don't want to keep them both, delete the one you don't need.
Addition: you can prevent to have both files in Dropbox by saving it with the same file name to Quickoffice. When you transfer the file back to Dropbox, it will ask you if you want to replace the old file. When you accept, the older version will be removed.
They are supposed to have added the ability to edit MS Office documents directly in the Google Docs, Sheets, and presentation apps; and that is their reason for discontinuing QuickOffice.
I have not tried this myself, and I have not seen any updated description info in Google's iOS apps, so it's possible these changes are still pending, or waiting for approval in the App Store.
That does not mean that you have to give up QuickOffice, for now, but you should be preparing for the day when you have no choice.
P.S. Unfortunately it does not look like the Google Doc apps will offer direct DropBox support. No idea if they will in future versions.
If you have the latest version of QuickEdit, which is, I think, a similar program, there is an open button top left of screen. Tapping that will show multi options, including local files on your iPad and Dropbox. Tap Dropbox and all your spreadsheets will be shown. Tap the one you want to open.
When finished editing click close at top left of screen and choose save.
If you want to save any file to a different location, e.g. A local file to Dropbox or vice versa, tap the button that looks like a floppy disk and choose a pocation.
It works a treat, and I use this program almost every day. Quick Office I found not so good. Good luck.