Flash is multipurpose, easy, flexible and feature-rich theme. It has drag and drop page builder that makes it more advanced. Flash is available in free and premium version. Free version does not allow you to change the footer credit text. To change the footer credits just add this code in your child theme ‘function.php’ or if you have any custom plugin you can put this code in your plugin.
If you have problem on editing your child theme or you don’t have any child theme, you can use this plugin. Just download and install this plugins and it starts working.
After installing this plugin you may able to edit footer text from theme customizer.
If you are using Spacious Theme by ThemeGrill then read How to Remove Footer Credit on Spacious Theme by ThemeGrill?