They have already answered this question many times (I wouldn't be surprised if they even put it in their FAQ). You can expect this answer: "Unfortunately the kind of upgrade you're asking for is not possible, however, you can cancel your 40EUR order and place a new one for 100EUR".
Q.1 I have purchased a 40€ pre-order package and want to upgrade to the 100€ package. Please help! A. Currently the upgrade path is as follows: Email Jolla that you wish to upgrade your pre-order package from 40 to 100€. In the email, request Jolla to cancel/delete your confirmed 40€ pre-order package. Then, please proceed to place a new order for the 100€ pre-order package.