Please don't listen to Doc J or other up there recommending you don't stretch your strings. BS!
There is nothing wrong with stretching the strings. I do it every restring and mine stay in tune perfectly after just a few days.
Here's how: with the uke in your lap facing up, tuners to your left hand. put your finger down on the first or second fret of the string you plan to stretch (say start with the G), then starting at the bridge end and with your index finger, pull upward on each string in several spot all the way up the neck. Do this several times per string, then retune. PLAY.
Then do it again. Retune and PLAY, play hard! lots lots of strumming, get those strings heated up!
Then do it again. Retune and PLAY.
Next day, retune. play. continue until retuning is minor and normal again. Usually only takes about two days for me.
I also do some advanced stretching of the strings that involves a door frame before putting on the uke, but I don't recommend that to just anyone.
Good luck!