If you create a document online, then upload a new version of that document from your desktop that is a new document.
The old document doesn’t automagically incorporate all the possible changes you could have made to the new version.
You have to manually transfer changes over.
Generally this is not something you want to do in the online stores basic code editor.
Instead download BOTH your live theme and target update theme to work on them locally.
And use a file difference merge tool like meld.
Teaching the step by steps of a specific theme update process is way beyond the scope of the forms.
If you are not familiar with theme development and need a theme update/migration then contact me by my email for services.
Contact info in forum signature.
ALWAYS please provide context, examples: store url, theme name, post url(s) , or any further detail in ALL correspondence.
Good Luck.