The replace filter or regex_replace can be used to replace data in a string or variable. The replace module is used to replace data in a file.

In this example, "Hello" is replaced with "Goodbye".  Notice in this example that Hello World is wrapped in quotes. When wrapped in quotes, Hello World is interpreted as a string.

- name: "Replace 'Hello' with 'Goodbye'"
  debug: 
    msg: "{{ 'Hello World' | replace('Hello', 'Goodbye') }}"

Which should return the following.

TASK [Replace 'Hello' with 'Goodbye']
ok: [server1.example.com] => {
    "msg": [
        "Goodbye World"
    ]
}

Let's say you are using the vars plugin to create a variable, like this.

vars:
  foo: "Hello World"

The debug module can be used to output the variable, like this.

- name: "output the contents of the 'foo' variable"
  debug: 
    msg: "{{ foo }}"

Which should return the following.

TASK [output the contents of the 'foo' variable]
ok: [server1.example.com] => {
    "msg": [
        "Hello World"
    ]
}

The replace filter can be used to update the data in the foo variable, like this.

- name: "output the contents of the 'foo' variable, replacing 'Hello' with 'Goodbye'"
  debug: 
    msg: "{{ foo | replace('Hello', 'Goodbye') }}"

Which should return the following.

TASK [output the contents of the 'foo' variable, replacing 'Hello' with 'Goodbye']
ok: [server1.example.com] => {
    "msg": [
        "Goodbye World"
    ]
}