1. Press windows key next to your left control + R
2. Search %appdata%
3. Go to kingdom_rush_origins then slot_1.lua (or whatever slot you want all heroes unlocked) and open it with a notepad
4. Press left control + f and paste ["levels"]
5. Copy and paste everything from [2] = { to [15] = { under the first level (do not include those spaces between text like in my guide)

[2] = {
};
[3] = {
};
[4] = {
};
[5] = {
};
[6] = {
};
[7] = {
};
[8] = {
};
[9] = {
};
[10] = {
};
[11] = {
};
[12] = {
};
[13] = {
};
[14] = {
};
[15] = {
};

Save this file and enjoy the game!