Hi Jerome.
You can do this using the ReportDocument object model. Create a variable of the type you need (e.g. TextObject) and then assign the report's object to it and change the Text property. Here is a sample for you (change the names accordingly).
Dim myLabel As CrystalDecisions.CrystalReports.Engine.TextObject
myLabel = CType(myReport.ReportDefinition.ReportObjects("Text1"), CrystalDecisions.CrystalReports.Engine.TextObject)
myLabel.Text = "your text"