You do nothing.
You were paid for your work. It's perfectly within the rights of the client to hire someone else. The only stipulation would be if artwork which you contractually retain the rights to was utilized outside the agreed upon usage.
I know discovering someone has adjusted something you've created can be troubling and sometimes upsetting. However, for client work, there's nothing which can really be done unless there's some legally binding agreement preventing revisions without your approval (which is exceptionally rare).
At most reaching out with a polite, cordial, inquisitive, email may alleviate some feelings....
"Hi! I noticed the product photographs on the web site don't display the final designs we agreed upon. I really like how the xxx was chosen for the flavors and I think the product shots look amazing. I was just curious if there was a reason you felt I wasn't the right person to assist in finalizing the labels?"
(Intentionally and honestly complimenting the new labels so as to avoid any connotation of animosity.)
That's about the most I'd do, without a contract breach.
There are really countless reasons such a thing may happen.. and many may have nothing to do with you or your work specifically....
- It could have been a purely financial issue (no clue about rates or pricing, this is a wild guess).. the client found someone cheaper to adjust things rather than paying your rates while you went back and forth about how to display the flavors.
- The client got some advise from someone well versed in product labeling (with an on-staff designer) about changes necessary. So they merely had that other designer implement changes.
- A last minute spark of inspiration about displaying flavors hit the client and they had a friend capable of making adjustments since they had your files.
Point is.. it is rarely a personal "snub" unless there's been some on-going personality conflict with the client. It was most likely a pure business decision.
From a design business standpoint, this is why native application files typically incur additional costs and are never provided for free. The ease at which adjustments can be made, if the client has native application files, is a commodity which has value. With native files, changes may take mere minutes... without native files, changes may take hours.