Continuing the discussion from how do i synchronise labels in writer:
I managed to get in the same situation with no [Synchronize labels] button. The suggested fix
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
doesn’t work in LO Writer 126.96.36.199. Npthing changed for me.
What worked for me was to select the first label, format it (I changed its font and paragraph indent), then choose Styles > Update Selected Style.