tl;dr: Just read the bold and the links.

So with the latest TS4 update we gained another hair category - updos. Unfortunately, Sims 4 Studio hasn't yet been updated to match, so there's no easy button to tick to put the tag in. But it bothers me that none of my CC hairs are using the tag, so I sat down, figured out how to tag them myself, and now I'm here to share just in case it bothers anyone else as much as it does me. Watch as the program gets mysteriously updated sometime soon and renders all my time wasted.

You'll need Sims 4 Studio, obviously, and some CC hair that needs retagging. Also, if you have a lot of hair that needs retagging, you need plenty of time. There is no fast way to do this at the moment, sorry.

This post assumes minimal experience with S4S, so I'll be explaining as we go.

1. Open up your CC hair in Sims 4 Studio. Switch to Warehouse view using the tabs at the top of the window.
2. Look in the big list-o-stuff on the left side of the window for the CAS Part for the black swatch and click on it.
3. On the right side of the window, scroll down until you see the Tags field. Click the button next to it that says "Edit Items...".
4. The window that pops up might be kind of intimidating if you've never used S4S before. It'll look different for different hairs, that's fine. At the bottom of the list it'll have four buttons; you'll want to click the one in the top-left that says "Add".
5. On the right side, click the drop-down box next to "Category" and look for HairLength. Click on it.
6. Now go to the field next to TagValueNumber, click in there, and type in 2173. Ignore the text that crops up in the box above.
7. Your newly-made tag should look like this. Click the "save" button in the lower right of the Tags window, then save your package.
8. Tadah! Once in-game, your CC hair will be tagged as an updo. Now repeat the process for all the hair you want to tag as an updo, and slowly die inside as you do it.

Please note: you do not need to tag all the swatches of a hair! Only the first one - which will, for most hairs, be the black one. You don't need to go through this for recolours that add extra swatches to the original hair.