I usually try to modularise the code to make it easier to maintain the userforms and add/remove elements on the userform without having to update the code as well. The following code assumes you have populated the Tag property of each textbox on your userform and then you can use that tag value to find the relevant bookmark or content control that aligns with that textbox.
Code:
Private Sub CommandButton1_Click()
Dim aCtrl As Control, sTag As String, strPath As String, aRng As Range
For Each aCtrl In Me.Controls
Select Case TypeName(aCtrl)
Case "TextBox"
sTag = aCtrl.Tag 'aCtrl.Name 'if you prefer
If ActiveDocument.Bookmarks.Exists(sTag) Then
Set aRng = ActiveDocument.Bookmarks(sTag).Range
aRng.Text = aCtrl
ActiveDocument.Bookmarks.Add Name:=sTag, Range:=aRng
ElseIf ActiveDocument.SelectContentControlsByTitle(sTag).Count > 0 Then
ActiveDocument.SelectContentControlsByTitle(sTag)(1).Range.Text = aCtrl
End If
End Select
Next aCtrl
strPath = Environ("USERPROFILE") & "\Documents\"
ActiveDocument.SaveAs strPath & Me.TextBox2 & Me.TextBox1 & ".docx"
Me.Hide
End Sub