Just test data and uptating one field using put:
document.getElementById('submitBtn').addEventListener('click', submitPost);
function submitPost(e) {
e.preventDefault();
const data = {petid: document.getElementById('petid').value,
species: document.getElementById('species').value,
_token: document.getElementsByName("_token")[0].value,
_method: document.getElementsByName("_method")[0].value};