You can use standard web components to let a user select a file to update. For example, you can place a button on a page that lets a user browse to the file to upload. You could also define a JQuery plug-ins to define a drop zone so a user can drag a file into the dropzone on the page and its uploaded to AEM. There are different ways.
See this community article that walks you through How To upload a file to AEM - is uses a button to browse to a file: