I'm not sure but I think the down button selects a closer target whilst up selects one further away. If not, it just sequentially selects all the mechs on the map, one by one.
Slightly more on-topic
Are there any techniques one can recommend for battles against multiple enemies?
Target button + Down does not select a closer target. Just confirmed it right now while in melee and targeting a distant mech. What Target + Down does is if you hold it down, it releases target lock entirely until you press it again.
Unless you're a grenadier, run. Even if you're a Cymbidium or XP-3, it's too risky to fight more than 3 at the same time in close combat, because you WILL get raped by the ones who aren't in melee. Fight them under your own conditions. If you've got area attacks like Iris's Magic Circle, 2000/XP-1's beam burst, or a missile swarm, you should get to a place where you can use them and still dodge incoming fire. However, if you're 2000, I recommend zooming around as a plane and launching rockets all the while (Alt weapon 1). Really, any mech where you can run away (not facing them) and still attack them has an advantage. XP-2 has a special case with the final weapon (massive butthurt missiles). They have a fairly large explosion radius, so if you can draw a couple of bots into close combat and dash out while charging your missiles (lulz), release them while you're still relatively close so they don't spread out again to attack you. You can do MASSIVE DAMAGE (weak point) to a bunch of enemies this way. With the same missiles, you can switch the target for the first and second missile. Just change target after the first has appeared but before the second appears. You do get some leeway, though, so you can still change the second target for a moment after the second has appeared.
Edit: I realized I didn't address Selena. Selena is also good in melee against multiple opponents due to its alternate attacks and massive swing radius.
Again these are just my strategies. They don't always work (I was up against 12 Blackcombs IN A ROW last night. Shit was NOT cash.), but maybe you'll get ideas better fit your own play style.
Edit again: I forgot to mention: VERY IMPORTANT. Know when to take a smaller attack to knock you down so you can dodge a deadlier attack with the invincibility periods.
Another edit (lulz): I just discovered that Anthurium has yet another dodging mechanism. Back Melee attack will cause you to lose all momentum (VERY useful) and begin to drop.