Changing FS landmass's?

Hello everyone.No doubt there is a specific forum more suited to this question, but if nobody can answer my question in here, then hopefully they can point me in the direction where answers might be availabe!I would like to know what program (if any) is available for changing elevation/landmass/landform data within flight sim? By this I mean that some areas are far from realistic with regard to terrain features such as ridges, valleys, mountains etc. Whilst this is understandable, given that Microsoft have modelled the entire world within FS, they could hardly be expected to have everything totally realistic, but I would like to change a small area for myself. So is there such a program available? If so is it freeware or payware? Also, how difficult is this type of scenery development considered to be?Hope someone can point me in the right direction.Best regards,Al Summers.