Fastest way to change all items in a nested list

Hi all,

So atm I serializing following json string into a list:

[
  {
    "Tag":"Tag1",
    "Categories":["A","B"]
  },
  {
    "Tag":"Tag2",
    "Categories":["A","B"]
  }
]

[
  {
    "Tag":"Tag1",
    "Categories":["A","B"]
  },
  {
    "Tag":"Tag2",
    "Categories":["A","B"]
  }
]

What would the fastest way be to change all category string to lower case? I'm using a nested for loop now after serialization.

Best regards!