Update style button not appearing in Pages This issue occurs for me constantly in Mac Pages. When I want to update a style, such as changing line-spacing or fonts, the "Update" button will not appear next to the selected style in the format section. Therefore I cannot make global changes to the style. 

Sometimes I am able to "force" the button to appear by changing line spacing to a different number then click Update, then change it back, then click Update again. But this does not always cause the Update button to appear either. The appearances are totally random.
I think this has to do with your current selection. I think Pages makes a distinction between paragraph and character styles, just like MS-Word (bleh). I seem to remember that in older versions of Pages this was a bit more obvious (are character styles even a thing in Pages anymore?). In typical Apple fashion, things seem to be handled "automagically" now.

I'm using v8.2.1 here, and I seem to be able to get the "Update" button on the style whenever the current selection is clearly "an entire paragraph". Line spacing will always be a paragraph-only property, so the "Update" button will appear, even if the selection isn't just a paragraph. Changing the color of a single word on the other hand, will not offer the "Update" button. But the button does appear when you select an entire paragraph first, and then change the color. 

I still keep a copy of good ol' Pages '09 around, because it has some features that are still missing in the rewrite (mirrored page margins for example). I'm not a fan of this type of "productivity" software, but Pages is the least annoying of the bunch (MS-Word being the worst, Google Docs way too limited for anything serious).