I wouldn't use the webui for that. Getting rid of onedrive in favor for a self-hosted nextcloud, I used the native client to download all the files on the machine and then moved them out. This also removed them from onedrive after acknowledging the "A lot of files have been deleted from your onedrive account" warning. Actually deleting the onedrive application was also not as straight-forward as some other users may want you to believe. Even now, I'm not sure it won't just pop-up one day once again.