Answered question
What should i do to change the plan or upgrade it?
If you have not activated your sim yet, you can choose any plan you want.
Other answers
Set up your plan "from scratch" on the website.
There's actually an option to buy the SIM card in advance and choose the plan later. You can just log into your account and choose the plan you want and then activate the SIM card using the code your received with the SIM.
the plan can be configured during the SIM activation process