Make sure the larger pot has holes in the bottom for drainage.
Lay newspaper on the work surface to make cleanup easier.
Fill the new pot halfway with a soilless planting mix. Add a timed-release fertilizer, according to label instructions, to the pot and use a stick or spoon to combine it with the planting mix.
Slip the zinnia from its old pot and gently shake it to remove the bulk of the soil from the roots.
Place the roots in the new pot on top of the planting mix. The zinnia should sit at the same depth as it did in its old container. Add or remove soil to get it to the proper height.
Fill the pot with the planting mix and use your hands to pat the soil around the base of the zinnia.
Water the planting mix slowly until water runs from the bottom of the pot. Allow it to drain for five or 10 minutes and then water again, allowing the pot to drain completely.
Place the potted zinnia in a sunny location and keep the soil moist.