At creation, RAW and RAI suggest you are limited to taking a single item, which you must do.
However, after your 1st or any subsequent match, you can then change that item into another set of items during the Rearm step where you get 100 points to play with as long as your Leader's not dead (ok, that's strangely an ironic phrasing there). That's what's described in the Resupplying a Tyranid Warrior Kill Team list, specifically with the example they gave, too, where they swapped out the starting Scything Talons.
We had a Tyranid player who was struggling with this and, after much deliberation on RAW and RAI, that is what he believes it to mean, and the rest of us think it is the closest adherence to RAW and makes sense RAI with the example they showed. The Tyranid list takes some forward planning.
Not sure if you got access to the book, let us know if you need more information on the Recruit/Rearm/spend your Cache step in the post-game.