public RelayCommand ChangeUC => new RelayCommand(e =>
        {
            UserControl1 uc = new UserControl1();

            Control = uc;
        }
 private ContentControl
 public ContentControl Control
 {
   get => control;
   set
     {
       control = value;
       OnPropertyChanged();
     }
 }

<Grid Margin="0 60 0 0">
            <dragablz:TabablzControl Name="Tabcontrol">
                <dragablz:TabablzControl.InterTabController>
                    <dragablz:InterTabController/>
                </dragablz:TabablzControl.InterTabController>
                <TabItem Header="HOME">

                </TabItem>


                <!--Problem is here-->
                <TabItem Header="Books" Name="DesignTabItem"
                         Content="{Binding Control,Mode=TwoWay}">
                </TabItem>
                <!--...-->


                <TabItem>
                    <TabItem.Header>
                        <materialDesign:PackIcon Kind="Login" Foreground="{DynamicResource PrimaryHueLightBrush}"/>
                    </TabItem.Header>
                    <TextBlock Text="github.com/Abel13/TabMenu2" HorizontalAlignment="Center" VerticalAlignment="Center"/>
                </TabItem>
            </dragablz:TabablzControl>



            <Button Height="50" Width="150" Content="ChangeUC"
                    Command="{Binding ChangeUC}"></Button>

        </Grid>