You can change the aliasing of one or more selected peaks by right-clicking on them and selecting Set Aliasing.... You can choose which dimension to change the aliasing in, and by how many times the peaks are aliased. Note that negative aliasing will move the peak to lower ppm values while positive aliasing values will move the peak to higher ppm values. The program will then move your peaks to the new unaliased positions and repeat the spectrum contours so that the peaks are not in the middle of nowhere.
If you double-click on a spectrum in the sidebar, it will bring up the Spectrum Properties dialog. In the Dimensions tab you can set the aliasing for each dimension manually. If you don't want aliased contours to be drawn, uncheck the Draw folded contours check box. If your aliasing is such that the spectrum is mirrored rather than cyclically repeated, select Dimension is inverted.
Peaks are shown normally in the position where they have been aliased to. In other repeats of the spectrum, they are shown with reduced opacity. In the Spectrum Display Settings you can select whether or not to view the aliased peaks or their labels, and you can change the opacity with which they are shown. You can also alter the defaults behaviour for these things in your main Preferences (File / Preferences or Ctrl/Cmd + ,).
Note that when you delete your unaliased peaks or middle-mouse button drag them back to the normal spectrum bounds, the disply will continue to show the repeated contours. You can remove these manually in the Spectrum Properties dialog by setting the Minimum/Maximum displayed aliasing back to 0.