Unfortunately (or fortunately, depending on your viewpoint) we cannot use tachyons for signalling to the past. I suggest the following as reading:
The Physics SE Question "Do Tachyons Move Faster than Light" and its answers
My answer to the Physics SE question "Would a tachyon be able to escape a black hole?"
Basically, for the consideration of faster-than-light signalling questions, tachyons are very like light in optical mediums in anomalous dispersion frequency bands where the phase velocity of the light can be greater than $c$. A monochromatic wavetrain with a greater-than-$c$ phas velocity has infinite length (like any monochromatic wavetrain), so it is meaningless to talk about signalling with such a thing. When you try to build localized signalling pulses from superpositions of faster-than-$c$ phase velocity sinusoidal waves the pulse travel / spreading speed is always $c$ or less.