Within your "Camp>Marbles" menu you can choose two marbles and attempt to upgrade the highest one to its next level by selecting two and then clicking .
You can only combine two marbles that are of the same kind (sickness or buff) and of the same benefit (type of attribute or sickness). For example, two Rot marbles can be combined, but a Rot and a Venom marble cannot be.
If it is not possible to upgrade a marble any further the system will halt the operation and tell you that there is no point to the upgrade, thereby preserving your marbles.
All fusions have a maximum success chance of 25%, which is decreased by the difference in rarity between the two marbles used. You can reduce this penalty by upgrading the Player Growth Attribute Enchanter .