Upgrading is a process where you have chances to strenghten your Hero.
It is a simple process, you will need just your hero and another hero card.
There are some things you need to consider.
Trying to upgrade costs gold. So save your money or farm stages to always be able to fulfill the process! Consider each upgrading range costs differently, +1 to + 2 costs far less than +4 to +5!
Chances depend on which hero you are going to destroy. You read well, the other hero you will use will be DESTROYED, so be carefull! (You don't want to risk to lose an high grade one, right?). Depending on the grade of the two heroes, you will have a different % of succeding. The more the grade of the heroes are similar, the more the % is near 100%. Look at this.
A 4-grade hero (orange) will have 100% chances to upgrade if another orange one will be used, 50% if the second one is blue/master, 25% if it is green/rare, 10% if white/common.
But don't worry, it is not all based on luck! Here the come upgrade points! Every hero has a number of upgrade points displayed in its sheet, going from 0 to 99. Those points are accumulated each time upgrading fails, and the number of points awarded depends on the relation of the two heroes you are using! For example, if you fails adding a 3-g (blue) hero to a 5-g (red) one, unique hero will gain 20 points!
When the point of current upgrading reach 100 (considering the one you are doing!) the upgrade is sure at 100%!
When upgrade succedes, your monster get a +1 and become more powefull!
Do not upgrade 1g or 2g heroes except in very first moements of your adventure, it is a waste! Better saving unused heroes and golds for +3-+6 heroes, the ones you will mostly use!