Trigger BeforeUpdate caused by: System.FinalException: Record is read-only

Question

Created one book object in that applying for update trigger on age condition if book type is horror ae should be updates as 21

Answer 1

You are trying to update old value through Trigger.old, thats why the error is.

Trigger.old is always read only.

Try to update new value Trigger.new instead of Trigger.old

Refer Context Variable Considerations