when ever you change the strings on your guitar they will need to stretch out before you can play them well
ill grab the string with my pick hand down by the bridge and then grab the same string with the fretting hand a few inches above the other hand and this hand holds the string down to the fret board. pull it with the picking hand away from the guitar a little bit maybe a 2 or 3 inches and with it pulled away from the guitar move both hands up together to the first frets while keeping the string the same way the whole time you slid it up with one hand holding the string down and right next to it the other hand holding the string away from the guitar a few inches
All strings need to be done like this but keep in mind that the D string is not as strong as the other strings because it has such a small core and you should not try to stretch that one as much
on the other hand the thicker strings like the G string and others need to be stretched out a few times
if the stretching does not happen then you will have to tune your guitar very very often even just pushing the strings to the fret board will cause it to get out of tune if the strings are not stretched well.
when i first started doing this i was not to sure and i would sometimes break a string because i was pulling it to far away from the fret board but make sure you only pull it as far as you would bend it like a full step bend any more than that is not good
Doing this will allow you to bend those strings like a mad man and still have perfect pitch after the crazy bends are over. stretching and a little graphite on ur nut. goes a long ways.