If it is in a pot, I assume you are keeping it indoors. When soil of potted plants gets very dry, it tends to shrink away from the sides of the pot. That does NOT mean it needs a larger pot. Simply tamp the soil back around the sides to fill in the spaces and then water a bit more frequently so it doesn't get that dry again.
If the main stem is not vertical, then gently re-position the rootball in the existing pot so it is vertical and no longer leaning. A slight adjustment is probably all that is required.
If the plant and pot are tipping over and straightening it does not solve the problem, then put the nursery pot inside a larger heavier planter (ceramic or terra cotta) that will provide the ballast needed to keep it from falling over.
