I try to make changes within theme but it doesnt save changes

Does your theme use Visual Composer? If that’s the case, check the version (update it to the latest or newer than 5.4.2)

Known issues

• Saving posts or pages is not possible in all browsers when using Visual Composer version 5.4.2 or older.

Plugins with known issues

• Contact Form by BestWebSoft will cause the plugin/theme editors to time out and not save your changes. Workaround: Disable this plugin to use the plugin or theme editors.

https://wordpress.org/support/topic/read-this-first-wordpress-4-9-master-list/#post-9687603