Content backup issue and crashed existing file.css

I’m new in the world of development and for the needs of my internship I need to create a wordpress plugin, so far so good!

the plugin is functional! I wanted to decorate it with an option menu to edit a single file.css! thing that codemirro could make possible !!

the only downside is that I can not save the content after modification …
Would anyone have the kindness to explain me step by step !?

Thank you in advance!!

here is the little bit of code that allows me to instantiate codemirror with the css that I have to modify and save

// Load css and init CodeMirror

jQuery(document).ready(function ($) {
$.get(’…/wp-content/plugins/simpleprint/include/sprint_printStyle.css’, function (data) {
wp.codeEditor.initialize($(’#simpleprint-textarea’), cm_settings);