There’s no built-in method what you’re looking for (merging slices). However, you can use append method like:
s.setGrade(append([]int{80}, s.Grade[1:]...))
If you had to update two grade ints then you could have done:
s.setGrade(append([]int{80,95}, s.Grade[2:]...))
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solved Index slice within Main func when using setGrade()