Kaweco has one main nib size that fits on most of their pens. It comes in two colours: gold, for their Classic Sport, and silver for most of the rest.
The swappable nibs will fit on:
Classic Sport (plastic)
If you get a spare nib, it often comes in either a plastic section (some of the Sport nibs, and most of the italic nibs), or a nib housing, which is smaller and just around the nib and feed.
I recommend putting your thumb on the black feed, and the first knuckle of your index finger on the nib. Pull straight out (no turning), and the nib and feed should come right out. Swap.
When you’re putting the nib back on the feed, there’s a slotted area that the nib fits right into, so you can’t get it wrong. The bottom of the nib sits right at the ledge where the two indented circles are (closest to the finger).
For most of Kaweco’s other pens, including their Al-Sport, Liliput, Special, Allrounder and Dia2, it’s even easier to swap nibs.
The nib housing will actually unscrew right out, and you can screw the new nib and feed back in.
You can still pull out the nib and feed from the housing if you ever want to get a really good clean, or if you receive a spare nib that is in the section instead of a housing that’s screw-in. This is most likely to happen if you order an italic nib, but sometimes it just happens.