👉 I want to replace the reward I chose with a different one
In this case, you first need to cancel your initial contribution.
Once it is cancelled, you will be able to contribute once again to the project, and select the amount you want to give and the reward you want to receive.
👉 I want to add another reward
In this case, you don't need to cancel your first contribution. You just have to go back to the project page and contribute once again, by selecting the reward you want.
👉 I want to give a bit more
In this case, you don't need to cancel your first contribution. To do this, just scroll down the rewards, select "Give what I want". You can decide the amount you want to give.