First, If you are a native English speaker, check your grammar. It's pretty bad. If you are not a native English speaker, but deal with native English clients regularly, you may want to have a friend or someone proofread some emails/communications.
As you have it, but proofread a bit:
Back in 2015, you wanted to sell the full version for desktop and only distribute a 'light' version for iOS. It was the cost of creating a new application for desktop which seemed to make you switch both versions to iOS. Unfortunately, it's simply not fair for me to complete the extra work of a desktop version, when we agreed I was to be responsible for only the 'light' or iOS version. While I understand your focus has shifted to a mobile application, my fees have not been updated. The project requirements have changed, but my compensation has not. My quote was a fixed price and based upon the information I was presented at the time. I hope you understand that I can't work on a project forever with a limited fixed quote based upon original scope requirements.
As I'd write it....
In 2015, I was approached to create a 'light' version of the application which was to be distributed for iOS. It was understood that a desktop version was not part of the project scope. It seems you are now asking for both an iOS version and a desktop version. Creating multiple versions of the application entails additional work and therefore additional compensation. The project requirements have changed. My previous quote was a fixed fee based upon the information I was presented at that time. As requirements change, it should not be a surprise that there will be additional fees to support additional requirements.
For the record, any quote or estimate I provide has a limited time frame stated in/on it. Such as "Valid for 30 days". Any business returning to you 2-3 years later and expecting you to honor an estimate is kidding themselves and trying to take advantage of you. I have never heard of any estimate or quote being honored by any business past 90 days, much less a year or more. If your estimates/quotes do not currently have a deadline date to them, add one immediately. Otherwise, you may continually run into this problem.
Some unspoken tips for "more professional" communication...
Avoid communications where you explain what they are thinking or feeling, i.e. "that made you switch
ed both" or "back when we were still thinking" -- speculating on what people think or feel tends to lead to arguments. Stick to just facts, nothing more.
Avoid stating anything about how you feel.... i.e. "simply not fair for me" or "I hope you understand" -- Comments which allude to your emotions generally do one of two things - either they show the client you are uncertain and they may be able to bully you into what they want. Or, they convey a sense of uncertainty and the client may lose faith in your abilities. Your emotions, especially unsavory emotions, should never be conveyed to a client, especially in written communication. Over the phone, it's difficult to not convey emotions. But in general the less emotional you appear to be, the better your client relations will become.