Originally, app's description is changed within a builder. Under the Settings -> Publish Info tab, selecting one of the platforms (iPhone, Android or iPad) will open a set of info fields. One of those is Description box.
This can be changed within a builder as many times as you want before you actually publish your app.
Once your app is published, if you wish to change your app's description, you have to do it manually through the store. It is advisable to change the description in the builder as well.
If you have the app published under your account, you can change this yourself. This is how it's done:
Google Play :
First you need to access your Google developer console which you can do here.
Logging in with your data should bring you to your administrative dashboard. Here you can see the list of your published apps. Find the one you wish to edit and click on it.
Now you've opened the menu where u can manage your app, click on the Store Listing tab.
Here you have a list of options you can change, one of which is Description. This is where you edit your app description.
Once you've done with editing your app description, make sure you click the save & publish button found at the top of the page and your app's description has been changed.
Access your iOS developer account dashboard through the link here.
Log in with your credentials and click on the Manage your apps icon.
In this menu, you can see the list of your published apps. Click on the one you'd like to change the description for.
In the bottom left corner you will see the Current Version of the app. Clicking on the same tab will take you to the screen where the app description is editable.
Fill this out with desired content and hit the Save button in the top right corner.This is how you change the app's description. If you have any additional questions regarding this, please let us know. You can write to us at this email: firstname.lastname@example.org .