If you plan to make risky changes in a virtual machine (for example, testing new software or examining a virus), take a snapshot before you begin to make those risky changes. If you encounter a problem, click Revert to return the virtual machine to its state at the time you took the snapshot.
If the first action you take causes no problems and you want to protect the virtual machine in its new state, you can take a new snapshot. You can have only one snapshot at a given time. When you take the new snapshot, you replace your previous snapshot. You do not lose any data.