Well, this is a tricky thing because I too am working with this and learning as I go. I can make two suggestions... one is better than the other though:
First suggesting (the worse of the two)... but a quick down and dirty solution that makes the terrain elevations slightly better than the default... is to get the SRTM 70-some-odd-meter mesh and install the tiles that you want. The website to it is here: http://simviation.com/fsdterrainsrtm.htm
Read through the author's instructions, and download his map of the world so you can see what tiles go to what rectangles over the earth's surface. It's better than nothing. The only problem with it (aside from the fact that there are better scenery packs out there) is that it is limited to under 60-degrees south and north latitude (absolute... positive or negative), so if you're hoping to improve Alaska, or any other area near either pole... you're out of luck. The good news, is that it is relatively easy to use, it's free, and it covers all areas around the world less than 60-deg lat absolute.
The better choice (suggestion) is the FS Genesis 38-meter-mesh. Someone else here on the forum turned me onto this just a few weeks ago, and it is pretty stunning scenery. I downloaded and installed the pack for California, Oregon, Washington, and most of Nevada, and yes... there is a difference.
You can either search the forums for FS Genesis and the freeware downloads will be there, or do a Google search for FSGenesis and it will take you to their website. You may decide some of their payware is worth paying the money for. The only problem I've found with it, is the freeware is mainly for the United States, and if you wish to venture outside the U.S., you will have to pay. But if mesh terrain is your thing, then you may find it worth the extra duckets.
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