Initially I used a
upload_filesize = 50M
If it's an INI file, then your specific requirements can be probably met by
ini_file module (http://docs.ansible.com/ini_file_module.html).
If your change is limited just few lines (or eve one), then you can probably look at
lineinfile module (http://docs.ansible.com/lineinfile_module.html).
Should you however need to change large number of settings, then it may be more beneficial to use
template (http://docs.ansible.com/template_module.html) or
assemble (http://docs.ansible.com/assemble_module.html). The main disadvantage of this approach is that you may need to review your template or fragments when updating your dependency.