How to remove special character from field and save it ?

Have you checked whether the data you're providing to the field is an array of null items? Appian will add the semi-colon as a separator character:

It is in array format

Then you need to re-think how to display the data. You have a list and are currently trying to display in a single field, so Appian is trying to be helpful by rendering the list with the semi-colon as a separator.

How to remove that semi-colon ?

Yeah, need more info and Stewart is (obviously) right on and you need to rethink your code.

I'm not sure why you're displaying an array in a single text field both the below code can work depending on your exact use case.

a!localVariables(
  local!myArray: {"",""},
  local!emptyIndexes: where(
    a!forEach(
      items: local!myArray,
      expression: a!isNullOrEmpty(fv!item)
    )
  ),
  {
    a!textField(
      label: "Concatenate your array",
      value: concat(local!myArray)
    ),
    a!textField(
      label: "Remove empty indexes",
      value: remove(
        local!myArray,
        local!emptyIndexes
      )
    )
  }
)

...or you display an array of text fields (or whatever the component is you're using), one for each item in the array.

Yeah, I'm uncertain on the use case too.

a!localVariables(
  local!myArray: {"",""},
  {
    a!forEach(
      items: local!myArray,
      expression: a!textField(
        label: "Display each index in a seperate field " & fv!index,
        value: fv!item
      )
    )
  }
)

Yeah, I'm uncertain on the use case too.

a!localVariables(
  local!myArray: {"",""},
  {
    a!forEach(
      items: local!myArray,
      expression: a!textField(
        label: "Display each index in a seperate field " & fv!index,
        value: fv!item
      )
    )
  }
)