Redefine or erase predefined unit

I am using MathCad 15 and am working on some simple calculations to get used to the MathCad environment and syntax since I am pretty new to using MathCad.

Unfortunately I ran into a seemingly small problem to which I can't find a satisfying solution:

I defined a function f(t) which I wanted to plot in a 2D-XY-diagramm.  Since t is predefined as ton (t=10^3 kg) I get an error trying to plot my function f(t) over t.

Is there any way I can redefine or erase t so I can use it as a variable to plot my diagramm? I know I could use any other variable, but it just bothered me that I cannot use t for time.

Thank you very much in advance!