wordpress theme update function

Hello,

While developing new wordpress theme to sell on envato market. is there any standard theme i can download to start with?

i mean i can make the front and sub pages. i can make it live on wordpress as version 1.0. and then i want to sell on marketplace like envato. So how i add theme update functionality ?

do i need to add some code in function.php ? for example after 3 months if i add more work on same wordpress theme and upload new copy to envato market. How my users will see and update the previous version to new verion?

please guide.