Let's say your managed nodes (e.g. target systems) contain /tmp/foo.txt, and foo.txt contains the following markup.

Hello

The replace module can be used to replace "Hello". In this example, every line beginning with Hello in /tmp/foo.txt will be updated to contain just the text World.

- name: edit foo.txt
  replace:
    path: /tmp/foo.txt
    regexp: "^Hello.*"
    replace: "World"

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

TASK [edit 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 [edit foo.txt]
ok: [server1.example.com]

The delegate_to module can be used to modify a file on the control node (that's your Ansible server), like this. In this example, the /tmp/foo.txt file on the control node will be modified.

- name: edit foo.txt
  replace:
    path: /tmp/foo.txt
    regexp: "^Hello.*"
    replace: "World"
  delegate_to: localhost