How to replace to apostrophe ' inside a file using SED

How to replace to apostrophe ' inside a file using SED

I have a file "test.txt" that contain the following
+foo+
+bar+
What I want to do is to replace them into:
'foo'
'bar'
But why this code doesn't work?
sed 's/\+/\'/' test.txt
What's the right way to do it?