Submit the evidence to them! I know some friends who work in dictionaries, and they are more than happy to do revisions on things like earliest printed usage of a word, and people frequently do write in providing exactly that kind of evidence.

https://www.merriam-webster.com/contact-us

Seeing as this one was so easily googlable, the following point will seem a bit silly, but the research performed by dictionary editors is only as good as the source material they can get their hands on. There may very well be earlier documented usages of many words that they define, but if they can't find those earlier usages ... well, the oldest usage at hand is the one that gets documented.