Sabrina, the Animated Series - Season 1 Episode 54 Hexcalibur

Sabrina and Salem destroy Zelda's old magic book that was written and signed by Merlin himself. They travel back in time to the middle ages to when Merlin first wrote the book but a problem occurs and they go to far up to when Merlin was 12 yrs old. Back then he was an amatuer at magic and his friend (King) Dude Arthur was terrible at magic so Sabrina and Salem teach them and soon they find the mighty sword of Excalibur which they need to use to defeat the evil forces that threaten Arthur from becoming King. In the end they win and Merlin gives Sabrina a new replaceable magic book for Zelda. She and Salem go back and put it back on the shelf but when they accidently destroy all of Zelda's other books too the adventure starts all over again.