Thanks you for trying our game.
We are going to add tomorrow better instructions for the combat system. We planned to add an ingame combat tutorial but we run out of time.
As for your questions:
When you reroll, you reroll each time all not locked dices and you can do this up to 2 times.
You can lock and unlock between rerolls as many dice that you want.
For ex. you can lock the 1st and 2nd dice after the initial roll and then use your 1st reroll to roll the rest (3rd-6th) dice.
After that you can unlock (for ex.) the 1st die and lock the 4th, 5th because they have better values and use your second reroll
to roll 1st, 3rd, 6th dice trying to get the best dice values.