If ActivePresentation.Slides(1).Shapes(1).Type = msoTextBox Then
ActivePresentation.Slides(1).Shapes(1).TextFrame.TextRange.Text = "some text"
End If

this will be a simple way to change the text. also u have text inside each
runs and paraformat.

Are you creating the box with code? If you are using the AddTextBox method
you can use something like

Dim otxtbox As Shape
'values are left,top,width and height
Set otxtbox = ActivePresentation.Slides(1).Shapes _
.AddTextbox(msoTextOrientationHorizontal, 0, 0, 200, 50)
With otxtbox.TextFrame.TextRange
'change these to suit
.Text = "Abcdef"
.Font.Name = "Arial"
.Font.Size = 12
.Font.Color = RGB(100, 100, 0)
End With
Set otxtbox = Nothing