You can change this by modifying the map layout that you are using. You need to find the correct layout xaml by looking in the .../config/layouts folder. In your layout xaml there should be a section for the HelpLinks. The line you need to search for will be <HyperlinkButton NavigateUri="{Binding Url}" TargetName="_blank" Foreground="{StaticResource AccentTextColorBrush}" >. You need to change TargetName="_blank" to TargetName="_self".
Each layout is dirrefernt so i can't tell you which line it will be in the xaml. if you can't find it just let me know which layout you are using and i will take a look at that specific layout.
Hope that helps..