You cannot edit tracking data from the main screen. Instead, you need to use Edit -> Manually change data... to open the data editor (or just press C). In this editor, you can select up to two cells at once by clicking them. Using the Insert, Shift and Delete keys, you can insert, shift or delete cells or links. Press C again to exit the view.
To insert positions, make sure you have no cell selected, and then press the Insert key. To delete that cell position again, make sure that you have only selected that cell, and press the Delete key.
You can also use the Enter and Backspace keys as alternatives for the Insert and Delete keys, respectively. Both keys perform exactly the same function, so depending on your keyboard layout, you might find it easier to use one or the other.
If you press Shift while having a single cell selected, that cell will be moved to your mouse position. The metadata and links of the cell will be preserved.
Undo and Redo functions are available from the Edit menu. You can also use the Control+Z and Control+Y keyboard shortcuts, respectively.
To insert links between two positions, select two positions at two consecutive time points and press the Insert key. Select two positions and press Delete to delete the link between the two again.
A nice shortcut exists where if you have selected one single cell, then place your mouse cursor at the next or previous time point, and then press insert. The program will then both insert a position at your mouse cursor and create a link from the selected cell position to the newly added cell positions. This can be used to quickly track cells.
There are also some tools to work with multiple positions at once.
Use Control + A to select all positions of the current time point. Use Select -> Select positions in a rectangle... to select positions within a rectangular area.
Use Alt + A/D to move the positions to the left or right, use Alt + S/W to move the positions up or down in the Y-direction and use Alt + Q/E to move the positions up or down in the Z-direction.
Use Ctrl + Delete to remove all selected positions. (The normal Delete key will only delete cells if you have only one selected, to prevent accidental deletion.)
For visualization purposes, you can give every lineage a color. By default, every lineage is black. You can change this either in the editor (Edit -> Set color of lineage...) or in the Cells colored by lineage viewer (accessed from the main screen, not the editor). The benefit of changing the color in the Cells colored by lineage viewer is that you directly see the colors, as this viewer displays every cell in the right color. The given colors are also used in the lineage trees (Graph menu in the main screen) and in the 3D visualizations with Paraview (see tracking formats).
Links cannot skip time points. If you insert a link between two cells that are not in consecutive time points, then additions cell positions will be created in between the selected time points.
Links are used to tell the program that two cell positions at different time points actually represent the same biological cell. Therefore, you cannot create a link from one cell position in a time point to another cell position in the same time point. If you try anyways, a so-called connection will be made. You can write plugins to do something with those connections. You can also automatically create connections for all cells within a certain distance of each other, see Edit -> Connect positions by distance...