That function doesn't really accomplish anything. It does not return any value to calling source.
Code:
Function GetData(intI As Integer, strS As String) As Variant
GetData = Split(Mid(strS, 2), "_")(intI)
End Function
UPDATE tablename SET [Level]=GetData(0, [fieldname]), [Name]=GetData(1, [fieldname])
Or eliminate VBA:
UPDATE tablename SET [Level]=Val(Mid([fieldname],2)), [Name]=Mid([fieldname], InStr([fieldname], "_")+1)
Name, Level, Split are all reserved words and should not use reserved words as names for anything.