You can change the function by entering the new definition in the toolbar. Most standard mathematical functions such as sin and cos are available, as are the constants e, Pi, and i. In addition, the functions Re(z) and Im(z) are available to extract the real or imaginary part of their argument, respectively. Here is a complete list of available functions:
Sine sin(z) Cosine cos(z) Tangent tan(z) Arc Sine asin(z) Arc Cosine acos(z) Arc Tangent atan(z) Hyp. Sine sinh(z) Hyp. Cosine cosh(z) Hyp. Tangent tanh(z) Inv. Hyp. Sine asinh(z) Inv. Hyp. Cosine acosh(z) Inv. Hyp. Tangent atanh(z) Natural Logarithm ln(z) Logarithm base 10 log(z) Exponential (e^z) exp(z) Abs. Value abs(z) Square Root sqrt(z) Sum sum(x,y,z) Real Part re(z) Imag. Part im(z) Conjugate conj(z) Argument/Angle arg(z) If if(cond,trueval,falseval)
The toolbar contains a field to enter your function, four check boxes to determine what type of curves to draw in the domain, and a variety of buttons:
The buttons have the following meaning
start drawing/redrawing cancel current drawing operation
to check off which curves are to be drawn and which ones not
shows dialog to define general options shows dialog to define lines shows dialog to define radii, arcs, and circles
zoom in/out/undo in the Domain (also available via right-click on domain) 
zoom in/out/undo in the Range (also available via right-click on range)
slider to determine drawing speed to better see which curve is mapped where
show online, searchable Help
You can zoom in or out by using the various zoom buttons on the toolbar or by right-clicking on domain or range. Also, if you right-click on domain or range you can choose to pan right, left, up, or down from the popup menu. To quickly re-center domain or range, simply double-click on a point. Finally, you can also zoom in by dragging the mouse over domain or range to select an appropriate rectangle. All zoom and pan operations can be reversed by clicking on the appropriate undo buttons in the toolbar or by selecting undo from the right-click menu in domain or range.
You can check how your function maps individual points by
holding down the CTRL-key while clicking once on the point in the
domain you want to map. The point and its image will appear as dots. To
remove points, redraw your graph by clicking the blue check icon 
on the toolbar.