After you have downloaded the package theme from Themeforest. Extract it on your computer then follow the below steps:
Open your browser and log in to your admin page. Then go to "Online Store".
Click the "Upload theme" button.
Then browse the folder you extracted theme package, enter the "theme files" folder. Choose a zip file corresponding theme style you want to install. Ex: bt-woodstock-watch-1-v3-0-0.zip, bt-woodstock-fashionable-v3-0-0.zip, bt-woodstock-electronics-1-v3-0-0.zip,...
Choose a theme file you want to install
Click the "Upload" button.
The uploaded theme appears in the Unpublished themes section. You can install a maximum of 20 themes in your store. To publish the theme on your online store, click Publish theme. Your current published theme will become unpublished.
And you can click the "Actions" > "Publish" to publish the theme. A published theme is the theme that customers see when they visit your online store. Your published theme appears in the Current theme section of the Themes page, and your unpublished themes appear in the Theme library section. If you publish a new theme, then your previously published theme moves to the Theme library section. None of your theme changes are lost.
You can click the "Customize" button to customize your theme settings from your Shopify admin by using the theme editor. The theme editor includes a theme preview and a toolbar that you can use to add and remove content, and to make changes to your settings. To learn about which settings are available for your theme, see your theme's documentation.