If you are not familiar with modules, check out Ansible - Getting Started with Modules.

The replace module can be used to replace markup in a file on the managed node (e.g. the target system).

If possible, it is probably much better to use role template files to update a file.

Let's say /tmp/foo.txt contains the following text.

Hello World
Earth Hello

In this example, Hello will be replaced with Goodby in foo.txt.

- name: replace Hello with Goodbye in foo.txt
  replace:
    path: /tmp/foo.txt
    regexp: Hello
    replace: Goodbye

foo.txt should now have the following.

Goodbye World
Earth Goodbye

The register parameter can be used to capture the ouptut of the replacment module.

- name: replace Hello with Goodbye in foo.txt
  replace:
    path: /tmp/foo.txt
    regexp: Hello
    replace: Goodbye
  register: out

The debug module can then been used to display the output.

- debug:
    var: out

If no replacements were made, something like this should be returned.

ok: [server1.example.com] => {
    "out": {
        "changed": false, 
        "failed": false, 
        "msg": ""
    }
}

If one or more replacements were made, something like this should be returned.

ok: [server1.example.com] => {
    "out": {
        "changed": true, 
        "failed": false, 
        "msg": "2 replacements made"
    }
}

regular expression (regexp)

Regular expressions can be used. In this example, only lines beginning with Hello will be replaced with Goodbye.

- name: replace lines beginning with Hello with Goodbye in foo.txt
  replace:
    path: /tmp/foo.txt
    regexp: ^Hello
    replace: Goodbye

If the file was successfully updated, the play should indicate changed.

TASK [replace lines beginning with 'Hello' followed by anything with 'World' in foo.txt]
changed: [server1.example.com]

If the file was not updated, the play should indicate ok. This is the expected behavior if there are no lines to replace in the file.

TASK [replace lines beginning with 'Hello' followed by anything with 'World' in foo.txt]
ok: [server1.example.com]