Pointing up is best you need to train it to be up teasing it up can help otherwise go see a doctor
If it has until now always pointed up, and is suddenly not doing that, you really need to go see a physician, and soon. The behaviour of your penis can be a harbinger for a lot of vascular diseases.
Want to spend some time wallowing in a Recommended Read? Pick one! Or two! Or seven!
One thing you don't mention, is your age.
As one gets older, it is more likely that an erection, even if perfectly solid, will not be above the horizontal. Just think how many times it's had to try since puberty to, say, 50 !!
If you're in your 20's, for example, this and this really worries you, try some exercises such as draping a towel - not too heavy(!) - and tightening the muscles in your perineum - area behind your testicles. You'll find that the erection tries to lift it. For once, an exercises that can be fun and pleasurable !!
Don't overlook Hera Teleia's sound advice though, if it's a sudden change.
Good luck
underwear..upward or downward