Is it possible to update a SSDT DB project from a database?

its bit delay in answer. I am using VS2017 Database project in which I have achieved this task by comparing a local database with database project once the comparison is over you can update the database by update button

Step 1 right click on the database project and click on schema compare item.

Step 2 select target -> select database connection option

Step 3 change source and target

Review Screenshots for more detail

I am going with compare solution :
Choose schema compare and make your database as a source and database project as a target then compare and update

see the this answer

Firstly your right, it is a horrible situation so work on improving it in the long term!

There are two things you can do, firstly you could use SSMS "Generate Scripts" to export all the objects and then use the import in SSDT to import from the scripts - this isn't greyed out.

The second thing you can do is manually bring the changes in using the schema compare in SSDT, you can set the database as the source and project as the destination and choose what you drop, update and import.

Ed