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.