[cka]

Or 'caw' if you want to change the whole word, even if cursor is in the middle somewhere. I read it as "change a word".

[theonething]

hmm.. I've always used 'ciw'. What's the difference with 'caw'?

[xigoi]

ciw only deletes the letters, whereas caw also deletes one surrounding space. It’s more useful for other operations such as copying (yaw).