Changing the title of form fields

I have a complex form where, when I choose an item in a lookup the tiltle of several input boxes must change.
How can I do it?

You can simply set Title property for the form field:

Do you mean I can execute a script handler with set property and change the property name?

Yep. You can bind the Title property to a page variable and change it when needed. It will be easier for you if you convert the form to template and use the events of desired component directly.