To me it it looks like normal "dishing" from a thin top...If it isn't cracked, just play it..To repair would require additional bracing of the top...a lot of work..good luck, those are nice ukuleles..
To my eye, does not look like a top end Kiwaya. Although it may be solid wood (their laminates and solid woods are very hard to distinguish with the normal eye), so not terribly low end, it has no adornments or signs of extra workmanship. For instance, the sound hole isn't even well sanded. So, lets assume its a $300 instrument (it is not on par with Kiwayas $400 to $500 instruments that I have seen or owned... Famous is, I think, their tourist grade sold in Japan brand name)... then only you can decide if you want a luthier to put in more bracing (which will stop the dishing but not necessarily correct it), potentially ruin the sound, and cost you over a hundred bucks, I'm sure. The only way to remove the dish totally is to replace the entire soundboard, and I can guarantee you that will be cost prohibitive for a uke of this grade (and if for some odd reason it's a rare uke, you'd not want to replace it and devalue it anyhow)
I hope you sent these photos and a description (she's want to know model # etc from soundhole label) to Hitomi Kato at Kiwaya USA as I mentioned to you in your other thread, Normagal. She's a jewel, and will reply, and will help you with your uke value. I love Kiwayas, have owned several, and Hitomi has helped me by email every time.
http://www.ukuleleunderground.com/forum/showthread.php?74522-Uke-repair
If it were me, I'd remove the strings for a couple weeks, humidify the uke greatly, then restring ( most soprano strings are low tension, so just avoid the uncommon high tension ones, and don't use concert or tenor strings), and I bet the dish will be improved. Unless it cracks, like sirfink says, don't pay to fix it. My two cents.
Another option would be to carefully drill into the bridge (or have a luthier do it) to convert this to a string-through bridge design. Done properly, won't affect the sound adversely, and will then have upward tension on the soundboard.
Good luck. I have a sweet spot for Kiwayas, and I'd love to see this uke playable without breaking the bank for you, N.