how do i synchronise labels in writer

how do i synchronise labels in writer. I have created a new document under labels and created first label, but I cannot find the synchronise button to copy onto other labels. Where is it?

You have to choose the third tab when generating a new label page:

Options :: Distribute :: [x] Synchronize contents

If you have not done so it is impossible to gain this graphic interface control button later, imho.

I uploaded a sample file including synchronizing mode here: How do I find/create a template for a new Avery label that isn’t in Writer?

You may not be able to get the floating button again but you can still synchronize the labels or business card content from the menu.

Tools > Update > Update All

This is in version Version: 5.1.6.2 of LibreOffice. YMMV in earlier versions.