Hello I'm a newbee here.

I've got a avi-movie were the audio and video are good in the beginning but after 1,5 hour the audio is seconds before the video.
Is it possible to stretch the audio or schrink the video a bit?

Thanks a lot.

yes, what you are requesting is relatively easy to do:
Select the clip in the timeline and right-click Ungroup Clips (Ctrl+Shivt+G).
Now the audio is independent from the video. Be careful, you might accidentally move it now. But that's what we want now.
Now for your concern: try it, right click: Edit Duration. See how long the audio lasts and how long the video lasts. Probably the same length. Good, then we don't know now exactly by how much we have to lengthen the video, or shorten the audio.
Now you can right click: Change Speed. There you have to try until it fits. If you change the audio, you could choose Pitch compensation, but with the minimal changes it is not necessary.
Try it and report how it worked out.
And: Have fun with Kdenlive.