When you import the Customization package, assuming the source code is provided inside the package (not as dll/pdb) file,

• from inside the imported package => Extension Library => Create New you will need to create a library.
• When created, click on “Bind to Existing” from the same menu and ensures it is pointing to the correct library. 
• Open the solution created by the above action in VS.
• from the left pane of the imported package select the “Code” and move all items that you want to Extension Library
• If you do all these steps correctly you will receive a notification in VS to load the files in your project.
• Now you have the code in VS and can modify and compile it.
• Compile the project and add the dll file of “YourProject” to the customization package from inside the package “file” in the left pane.
• When modified and compiled you will need to refresh your page(s) to see the change result.

Also, I suggest watching T190 which will provide answers to your similar questions and an overall view of Acumatica customization.

Thank you @aaghaei. “When you import the Customization package, assuming the source code is provided inside the package (not as dll/pdb) file”. This point is little bit unclear to me and all other points are well clear. That means the sender should add the source code to the package before sending the package. I can not find the source code yet in this received package. So can you please tell me to where the sender should attach the source code file and how, before exporting the customization project package?

As mentioned in my earlier comment, if there is any source and it is provided along with the package, you will see it inside the Customization Package under the “Code” in the left pane. If there is Code but provided as binary then inside the project Under the “File” in the left pane you should have a dll/pdb file.

In the below yellows are places where you can put code inside the Package and the red is if you have any binary files.

I strongly suggest you watch and read through T Series courses starting with T190 if you want properly perform customizations without breaking things. Even if you are a professional programmer Acumatica Framework is going to be very new and I believe academic learning is always better and more effective than trial and error. 

Cheers and Good Luck!

I do not see anything under Code and DAC in your screenshot. Can you send a screenshot of the “Files” of your Project? I see there are 4 files.

Thank you @aaghaei .I’m currently following the T series courses sir. Codes have moved to a extension library and DAC changes are doing in the VS in the instance that developing this customization project. Now I’m testing the project migration steps. So far I have migrated it to a test instance and all the developed features are well performing in the test instance. Then I wanted to develop some things in the test instance. This is the reason. 

From the screenshot as I suspected you are not given the VS source code. If you want to modify the code, you need the source code for the DLL file in your project.

I thought that after adding the .dll files source code file also added to the package. So I thought that not need to add the source code files separately. If that the case, can you please tell me how to add the source code files?

Hi @malinthawarnakulasooriya08 were you able to find a solution? Thank you!