Is it possible to use child theme of child them?

I am using iStore free WordPress theme for a website. It is a child theme of MaxStore.

Now I need to modify the iStore themes. Since iStore is a already child theme, I don’t know that is it possible to create a child theme of iStore ?

Is it possible? If not what is the alternative way to change child theme?

Is it possible?

No. There’s no such things as grandchild themes.

If not what is the alternative way to change child theme?

Many things can be done from plugins. It could be from very easy to very hard depending on how things are actually done in the theme.