Open Cubase by double-clicking the desktop icon. If you don't have a desktop icon, click "Start" and select "Programs." From the programs menu, select Cubase.
Click "Open" from the left hand "File" menu. Select the track with the beats that you wish to slow down.
Isolate the drum beat tracks. If you only have drum beats in the track, this is unnecessary. However, if you have other instrument layers you need to either mute them, or solo the drum beats. Both the "Mute" and "Solo" buttons are located on the left of each channel strip. If there are numerous drum beat tracks and one non-drum track, it's quicker to mute the one that you don't want to hear. If there is one drum track among multiple other tracks, it's quicker to solo the drum track, which mutes all other tracks.
Double-click the number box at the bottom of your screen. The number represents the global beats per minute, or "BPM" of the track. This is the tempo. The default BPM for Cubase is 120. Type in a number 10 lower than the current BPM and listen back. Continue to reduce the BPM by 10 until your beat is at or near the desired tempo. Change it by a single beat per minute if necessary, for example from 150 BPM to 149 BPM.
Open the Cubase step sequencer from the "Tools" menu. This brings up a visual representation of the drum beat.
Drag the second note in the beat to the right. Moving it right increases the time between the previous beat and the next one. This process can be quite complex, but if you recorded other instruments and therefore wish to retain the track BPM, this is the most suitable approach.
Move the third note in the beat to the right by the exact same amount. Use the step sequencer grid as a guide. Repeat this process with each note in the beat.