Yii2 Add ActiveRecord Model Attribute

In Yii2 I have an ActiveRecord model already created, based on a MySQL table, but I had to add a new column to the table, and I don't know which change should I do to the Yii2 model in order to be able to use this new column.

I tried adding "public $columnname" to the class, but it allows me to see the value on the column, but I cannot modify it (maybe there is something missing, like a "setAttribute" function)

What do you think?