See page 54 in the ACL manual for Reducing and Increasing the length of objects. Move the mouse to the bottom corner and hold it until a double arrow appears. Then drag the corner inwards the distance that you want.
Alternatively, put the mouse in cut mode by clicking on the scissors the clicking on the point where you want to cut. Then click on the object part to be deleted and delete it.
I have ACL 2013 and I had a previous version; both came with a pdf format manual. If you bought ACL on-line, then there would have been 2 downloads in the email that they sent you, one for the program and the other for the manual. Take a look if you still have the original email. If not, and if you have ACL 2013, the program may have installed a shortcut to the pdf manual at: C:\Users\Public\Documents\MAGIX\Audio Cleaning Lab 2013\Documentation. The original would be at: C:\Program Files (x86)\MAGIX\Audio Cleaning Lab 2013 and the file name is: CleaningLab_EN.pdf.
If you still can't find it, click on Help on the menu line at the top of the program and select Help. This will open the help program that basically duplicates the pdf manual. Click on the tab Contents and select Arranging in the master track, then Reducing and increasing the length of objects.
You can read all about the program by going through the Contents and selecting topics and subtopics.