The table has two keys: filename (partition key) and eventTime (sort key). 
I want to update eventTime for certain filename. Tried put_item() and update_item() sending the same filename with new eventTime but those functions add a new item instead of update. 
What should I use for that purpose?
According to DynamoDB/AttributeValueUpdate aws docs:

You cannot use UpdateItem to update any primary key attributes.
  Instead, you will need to delete the item, and then use PutItem to
  create a new item with new attributes.
DynamodDB: How to update sort key?