Hi @Aveeva,
Managed products bulk attributes option is the only way from the admin panel.
or you can update from database using mysql query.
In this case you need to find out the attribute id for "PreBook" attribute from eav_attribute table.
then you need to update catalog table with attribute_id and entity_id and store_id using update query.
catalog_product_entity_text (if your attribute is textarea)
catalog_product_entity_varacher (if your attribute is text)
I will not suggest using Database query if you are not expert in the database maupulations.
You shuld update using admin panel, or may be in bunch if taking too long time.
I hope it will help you!