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To modify the value of a pre-existing PostgreSQL column created using Type-ORM Nest JS, you can follow these steps:

1. Define a new entity class using Type-ORM decorators and specify the table name associated with it.
2. Use the decorator @Column to specify the column you wish to modify and its properties.
3. Import and use the TypeORM QueryBuilder to retrieve the data from the table and modify the required column value.
4. Use the TypeORM EntityManager to save the changes to the database.

Here's an example code snippet that illustrates how you can modify the value of a pre-existing PostgreSQL column using Type-ORM:

import { Entity, Column, PrimaryGeneratedColumn } from 'typeorm';

@Entity({ name: 'my_table' })
export class MyEntity {
  @PrimaryGeneratedColumn()
  id: number;

  @Column({ type: 'varchar', length: 50, nullable: false })
  my_column: string;
}

async function modifyColumnValue() {
  // create a query builder instance
  const qb = getRepository(MyEntity).createQueryBuilder('my_table');

  // retrieve the entity with id = 1
  const entity = await qb.where('id = :id', { id: 1 }).getOne();

  // modify the value of my_column
  entity.my_column = 'modified value';

  // create an EntityManager and save the changes
  const entityManager = getManager();
  await entityManager.save(entity);
}

In this code snippet, we have defined a new entity class MyEntity using the Type-ORM decorators. We then use the createQueryBuilder method to retrieve the data from the table and modify the value of the my_column column. Finally, we use the entityManager to save the changes to the database.