Try to get a loan and buy it from the owner. Caveat: Make sure you are paying for what it is worth for.
If you really like the house, seek the service of a realtor to guide you in the process.
Best of Luck.
In practice, why would you do this? Are you planning to take title to the property, as well? Is the property worth taking over the mortgage for?
You should contact a local attorney to have them review the mortgage documents before doing anything.