You can create a Font datatype and set this to the FontFace property:
function setItalicized(textLabelOrButton, isItalicized)
local family = textLabelOrButton.FontFace.Family
local weight = textLabelOrButton.FontFace.Weight
local style = if isItalicized then Enum.FontStyle.Italic else Enum.FontStyle.Normal
textLabelOrButton.FontFace = Font.new(family, weight, style)
end