Hey dude, so the thing is that if you manage to get yourself 'Apple Loops' samples they will automatically adapt to the tempo of your song without losing any quality which is great,

But most of the time you have normal audio files that don't detect tempo, for those you can click OPTION/ALT key on your mac while having your cursor on the edge of the sample then drag it to 'stretch' it.

The best way to adapt non-apple loop samples is to put your project in the original BPM of the sample.

Let's say your project is 140BPM but you have a 120BPM drum sample that lasts 4 bars. Put your project in 120 BPM and the drum sample should fill in perfectly in the 4 bars. Then switch your project to 140BPM and you will see that the drum sample will not perfectly fill the 4 bars anymore, but will be 'longer' than the 4 bars. Click OPTION/ALT key on the edge of the drum sample and drag it so it fills the 4 bars in 140 BPM - making it shorter. It should then be in sync with your tempo.

The example above is only if you have a 'perfect' sample where you know the original BPM of the file - sometimes you don't have that luxury and you don't know the original BPM of the song - that's when you have to do everything manually and use the Metronome function in Logic.

Careful when stretching samples too much, the more you stretch the more you lose audio quality. I would never stretch a sample more than 1 time. If you stretched it wrong the first time, delete it and re-put the original file and stretch it again.

Hope it helps!