To turn off smart quotes in Libre Office Writer, so that the double quote character is shown in the document as ” — exactly as you typed it — and doesn’t get converted into something curly:

• Go to Tools > Autocorrect Options
• Select the Localized Options tab
• Click the button under Start Quote. If you’re using the Basic Latin character set, scroll all the way to the top of the character set display and click the ” box (third box from the top left row, next to the !). If you are using some other character set try searching near the top of the set for the ” character.
• Click OK
• Click the button under End Quote. Scroll all the way to the top of the character set display and click the ” box (third box from the top left row, next to the !)
• Click OK
• Click OK

Smart quotes are now off for the document that you’re working on. They will also be off for any new documents that you create, including spreadsheets and files created by other Libre Office applications.

To get rid of smart quotes already in a document:

• Highlight any start smart quote and copy it (Ctrl-C or File > Copy)
• Select Edit > Find & Replace
• Paste the smart quote into the Search for box
• Type ” in the Replace with box
• Click the Replace All button
• Click Close
• Repeat these steps using a copy of the end smart quote