This blog explains how to customize the base app of business central (old fashion) .

Please Don’t customize the base app, Use extension wherever possible

Step 1 :- Copy Base Application from installer

Step 2 :- Unzip in designated folder .This contains app.json file where in target set to onprem

app.json will look like this

Step 3 :- Download the symbols

Step 4:- Add the following in either user settings/workspace settings . This is required for .NET DLL used in application

Step 5 :- Unpublish the base application and dependencies or else you will get an error stating that extension already deployed on tenant

Use following PowerShell command

Unpublish-NavApp -Name “Base Application” -ServerInstance BC150

Step 6:- Import license file and publish the modified app.

Hope this will help you.