Each pdf object has simple value or set of values which can be changed. Almost all pdf operators have operands which are normal pdf objects which can be changed. Implicit changing of object types is not allowed (e.g. integer object to string etc.) but this can be done by deleting and adding desired property type. When changing integer object only integers are accepted, when changing boolean values only true/false is accepted etc.
Editing a property
When editing a reference number a dialog appears which verifies whether a pdf object with specified reference number exists.
Text selection
To make a change visible perform these steps
edit the object value
press enter or click away from the property
reload page if the change is visible