In order to remove underscore from flutter app name (eg. app_name) do the following steps and you will get your app name like (eg. app name)

For Android:

Go to: android\src\main\AndroidManifest.xml and change:

android:label="app_name"

to

android:label="App Name"

For IOS:

Go to: ios\Runner\info.plist and change:

<string>app_name</string>

to

<string>App Name</string>

That's all.