As Moosetek says you can use ships to move pop from high pop to low pop worlds or you can do things to increase either that particular planets approval or your overall morale ability (i.e. Native Ability).
Or even more simply, you can do nothing. Once a planets approval hits 40% the pop on that planet stops growing and the planet's approval stops declining. The only real downside of low approval is that if you have a higher form of government then you may lose an election. However you can keep track of upcoming elections under the Government & Ethics tab in your Civilization Manager screen and simply reduce taxes during the week of the election.
However the intertwined relationship of population, taxes, morale and approval is central to this game and is far more complex than can reasonably be discussed in a single reply. You definitely need a deeper understanding of this topic to advance in this game. A good place to start is the GalCiv2 Wiki.
You should probably read through the above links. Undoubtedly you'll have more questions so feel free to ask, but as a start you need to be very careful about building any farms on your homeplanet or more than one farm on any colony.
After reading the above links the key thing to understand is that a planet's "base approval" is solely determined by the planet's population. The base approval is expressed as a fraction that gets smaller as the population increases. The importance is that the base approval also multiplies all other sources of approval such as entertainment buildings on the planet and native ability. So as a planet's population increases the base approval decreases which in turn decreases the effectiveness of all sources of approval increases.
For example a Virtual Reality Center (VRC) gives a 40% morale bonus. However the net effect to approval is determined by base approval which from the Wiki article is 0.91 at a population of 5B, 0.4 at a population of 20B and 0.2 at a population of 25B. So a VRC gives a 36% approval boost to a 5B planet, a 16% approval boost to a 20B planet and only an 8% approval boost to a 25B planet. Also note that the base approval numbers in the Wiki article are for DL and the DA numbers are even worse. I think in DA it's 0.2 @ 20B and 0.1 @ 25B. Plus in DA there's a cap on the net effect of native ability (i.e. the product of base approval times native ability is limited to 100%).
This makes getting a planetary population over 20B virtually impossible.
Like I said this is a very complex topic.