Can we change values in after trigger in salesforce?

Hello,

An AFTER trigger is typically used to update something other than the row being updated. For example, if you wanted to log the fact that an update had been made, an AFTER trigger is ideal.

To change the value of a column as it is being inserted, you need to use a before trigger. For example

CREATE TRIGGER modify_column BEFORE INSERT ON mytable
SET @column = value;

Where value is a query, pre defined value or NEW.column