Apologies in my haste I missed the realm from the request path:
/api/{realm}/asset/{assetId}/attribute/{attributeName}
Here’s a curl example for an attribute called humidity on an asset in the master realm.
curl -i -X PUT \
-H "Content-Type:application/json" \
-d \
'50' \
'http://localhost:8080/api/master/asset/5syzbGGCxe4ZZFvRNdjYJF/attribute/humidity'