Adding the alter table sciprts and running on SSDT solution

I have an exisiting SSDT solution and I want to add up the alter table scripts in the solutions and running on the SSDT solution such that the tables are updated according to the sciprts?

Example: if I already have a table student with two coloums sid and sname and I have a script which add up new coloum city to the table How can I do this such that table is updated?