How do I change a specific string or called char to another?

it really is quite simple, you just use string.gsub()!

read more here: strings

code:

local Alphabet = {"a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r","s","t", "u", "v", "w", "x", "y", "z"} -- just incase you wanna do it easier for decode and encode.

local Encode = {}
local Decode = {}

local function CreateEncode(One, Two)
   Encode[#Encode + 1] = {O = One, T = Two}
end

local function CreateDecode(One, Two)
   Decode[#Decode + 1] = {O = One, T = Two}
end

local function EncodeString(String)

  local Temp = {}
  
  for i = 1,#Encode do
    local Split = string.sub(String, i, i)
    Split = string.gsub(Split, Encode[i].O, Encode[i].T)
    table.insert(Temp, Split)
  end
  
  return table.concat(Temp, "")
end

local function DecodeString(String)

  local Temp = {}
  
  for i = 1,#Decode do
    local Split = string.sub(String, i, i)
    Split = string.gsub(Split, Decode[i].O, Decode[i].T)
    table.insert(Temp, Split)
  end
  
  return table.concat(Temp, "")
end

CreateEncode("a", "b")
CreateEncode("b", "c")
CreateEncode("c", "d")

CreateDecode("b", "a")
CreateDecode("c", "b")
CreateDecode("d", "c")

print(EncodeString("abc"))
print(DecodeString("bcd"))