Open iCloud.com in a web browser on your PC
Sign in with your Apple ID
Click Notes
Open the note you want to save
Select the note text
Copy the text
Paste it into a document on your PC
Save the document to your computer
For attachments, open the note and download each file separately if available
If Notes sync is enabled on your iPhone, make sure the notes are stored in iCloud
If needed, install iCloud for Windows and enable Notes access through iCloud.com in your browser