To improve singing there are some exercises that you can do:

• Proper deep breathing is key.
• Excercise because it opens up the lungs and stretches the diaphragm. For example, hiking.
• Lip flutters. Babies do this a lot, really cute.
• Drinking plenty of water and chamomile tea.
• Breathing exercise: Alternate nostril breathing. Plug one nostril with your finger; Inhale through the left and exhale through the right, then inhale through the right, and exhale though the left, and so on. 
• Inhale for 5 seconds, hold breath for 5 seconds, exhale for 5 seconds, hold breath for 5 seconds, inhale for 5 seconds, and so on. Keep breath steady and calm. Don’t manipulate or force. Go with the flow. Be kind to yourself.