Edit a dtsx through SSMS

No, I'm afraid not. The only way to edit .dtsx files is with SSDT/Visual Studio.

The only way to edit .dtsx packages is using SQL Server Data Tools for Visual Studio (Business Intelligence Development Studio in older versions)

• Download and install SQL Server Data Tools (SSDT) for Visual Studio

SQL Server Management Studio can manage and schedule package execution it is not where Packages are edited.

There are some configurations that you can edit in SSMS before executing the package like changing connections, passing parameters values, assign variables values and some other option only.

I managed to do a very small change simply by opening the file in a text editor. But really just a minor change in the Query used before transfer. I was lucky to find the row to edit by searching "WHERE".