If you want to move an attachment to another page, you need the "Add Pages," "Add Attachments," and "Delete Attachments" permissions from your space administrator.
To move the attachment, click on the three dots in the page menu, then click on "Attachments" to open the Attachments dialog.
Next to the attachment's name, select "Properties,"
in the "Page" field, type the name of the target page to which you want to move the attachment.
Then click "Save" and the attachment will now be moved to that page.
If the new page is in a different space, type the unique space key and a colon before the name of the target page.
You can watch our Video #23 of the Video Tutorial Series "Learn Atlassian Confluence" and also read about this in Atlassians documentation.