A recent attempt to replace a string within double quotes "string value" while using sed resulted in much trial and error. A quick Internet search revealed a working answer. It is here for my reference.
I wanted to replace "this value" with "that one" in a file, we'll call testfile. According to multiple sources, the sed command allows the use of other characters instead of /.
sed 's#"this value"#"that one"#g' testfile
This also works.
sed 's/\"this value\"/\"that one\"/g' testfile
Usually the man pages are useful, but for sed, the info pages are more informative. Below is the relevant selection for this article around line 107.