Download THIS application first.
Download Cheater from Above link. Its an xml editor, it changes hidden prefrence, value and setting of games and applications.
So open it look for HTR high tech racing in the list. click on it, then choose com.qubite.htr.
Here you can Unlock all the Upgrades, without spending single coin. Look for titles, Motor, chassi, Rodda etc, these are filled with something like |True|False|False etc. Change all to True. And everything in Game is unlocked
If You still want coins( never hurt for future insurance) Look for garnna column. Change it to your heart content. click back and choose yes to modify save.
If You want to unlock each track with Gold. Change all the colcocoes from 0/1/2/3 to 1. Here 1 is for Gold and 2 is for silver so on.
Some times You need to save after each modification. Anyway, click on run app and enjoy you new rides