Making it a strategic priority and carving out time for the team, to upskill each week. Is going to be a big help in fast tracking the development of skills. The team is ALWAYS busy, so giving them permission to carve out time from that, to go and improve, is invaluable.
In tandem you could also consider getting them a mentor, whether internal or external, to help them develop. And investing in outside education, whether its the local university, meetups, conferences or online courses. A combination of all of these, will greatly help to improve their skills.