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Hi, I'm Benjamin. Welcome to our site. I'm reviewing your question now and I'll reply with more details ASAP.
I want to make sure I provide you with the help you are looking for.
My understanding is you have an existing install of Ubuntu and you are trying to upgrade from a lower version to 16.04, is that correct?
Can you give me any more information so I can give you a better answer? How are you starting the upgrade and what error do you get?
I did find this information that could solve your issue, please do let me know your technical level of expertise so I don't skip steps or give you information you already know.
If that link is enough information to get your upgrade complete, awesome! In that case, I would greatly appreciate it if you would rate me! If not, just reply here with more information and we'll work together to get your upgrade completed.