You can transfer your existing notes from Apple Notes to UpNote by performing the following steps on Mac and Windows app:
Export your notes from Apple Notes to Markdown (you may use the external tool Exporter to get your notes exported to Markdown. However please note that Exporter doesn't keep all text format hence some formatting may be lost when imported to UpNote)
Select the option to import from Markdown files
On Mac: Select File > Import Notes> Markdown from the menu bar.
On Windows: Go to Settings> General>Import from Markdown
You will see the pop up as below:
Click the button Select .md files/Select folder and select .md files/folder to import to UpNote.