Go to yor Themeforest account and then navigate to downloads section and click on download button of the Musicplay Theme and download the package ‘Installation WordPress Theme’ file which contains only theme files no PSD files are included in this file.
If you download the full package you can find the Themefiles directory from which you can find the latest version released folder says example 2.0. in that you can find the relevant theme folder. It depends on how you wish to update the theme via FTP Cllient or WordPress Theme updates method.
Before you update the theme everytime you need to backup your current theme folder. Take a backup of your modified files first and make a note of what changes you did in the files so that you can replace the altered code later as it will get replaced when you update the theme. If you are using the child theme then you don’t have to worry as you can simple replace the parent theme folder without any hesitation.
If you wish you can also take backup of your theme options from Appearance > Theme Options > Import/Export
Upload via FTP
Go to wp-content > themes location and backup your theme ‘musicplay’ folder by saving it to your local computer hard drive, or you can choose to simply delete it. You will not loose any of your content or posts.
Retrieve the musicplay.zip file from your new Themeforest download and extract the file to get the musicplay theme folder.
Then simply drag and drop the new musicplay theme folder into wp-content > themes location. Choose if it prompts to Replace the current one if you did not delete the exisiting one.
Added:If you are using the child theme then you need to make sure you take a backup of the child theme as well and then update the files and we will not update anything in the child theme style.css file. Only version number will be changed in the style.css file of the child theme.
If required we will make a modification in child theme and for that we will make log of which file modified in the child theme as well.