Copy data from Dropbox to Nextcloud

I’ve setup Nextcloud 17.0.2.
My Windows PC has a small harddisk (256 GB) and runs with a large Dropbox as my current cloud.
What is the best way to copy (for this moment I do not want to move) my data from Dropbox to Nextcloud while both applications are syncing to my harddisk?

Best regards,
Guido