If you place down a note, and want to change the rhythm, hover over the note, and press [ to decrease the duration, or ] to increase the duration. Or you can multi select notes or measures in mass. Very cool! But we have different types of rhythm changes for you. See diagram below on how to access these options.
Types of duration changes
Absolute: Change rhythm of note "in place". Rests will be added to accommodate.
Magnetic Measure: Notes will stick together, just within each measure.
Magnetic Global: Notes will stick together globally. Use this sparingly.