"update on" and "update of" are both correct, but have different meanings. We also have "update for": update on sth = recent news about sth "The reporter gave us an update on the changing political situation in Syria" "Can you give me an update on the plans to improve our company's services?" update of/to sth = a change to make sth more modern "The play is an update of classic Chinese opera, bringing traditional ideas into the 21st century" "Learn how to get the new upgrade to Windows 10 by clicking here!"

Actually, they are all correct. Update accepts different prepositions adn it really depends on what is being updated. "On" is the most usual, it means there's new information: Here is an update on the issue that I brought up last week. -> I'm going to tell you something new about this, new information. While the rest of the prepositions usually refer to a physical, detectable update: Here is an update of the report for your reference. -> There is a new report. Also: An update for/to the software, an update for Jessica.

All of the examples are correct. An "update to", "on" or "of" are all correct. The thing that you should notice is, "update to" and "update of" both refer to a concrete object. Meaning the object is not abstract, you can hold it, touch it, feel it. "Update on the issues" is not a physical thing. These are my quick observations. (the preposition "to" usually refers to some location and "of" is like shows some kind of possession. Update of the software ... and update to the software ... are both correct.

Thank you for pointing out the error!

in the first sentence, the verb send should be "sent"