how do I rename tooltip of echart download icon

You can use the title to rename the tooltip, for example:

toolbox: {
    show: true,
    feature: {
        downloadTable: {
            show: true,
            // Show the title when mouse focus
            title: 'Save As picture',
            // Icon path
            icon: '/static/img/download-icon.png',  
            option: {}
        }
    }
}