Some KMC links are re-usable and others are "one shot"Conclusion: the only safe method for 9 speed and up is to use quick links, (and don’t forget most of these are ‘one-shot’ only and should not be re-used).
An alternative, if you can find it, is to have your chain cut to length from a bulk reel (some bike shops can do this) and use just a single link.
<Safety note ends>
"Connex chain" quick links are made to be re-used, you don't need a tool either and they're supplied with the chain.
Don't use an old quick link with a new chain (that's asking for trouble).