Says it works on spells 1 to 5th level. What if I cast a spell at higher than its level like a 9th level fireball?

9th > 5th, so doesn't work.

A spell when cast using a higher slot becomes that level. A spell that is normally 1st level cast using 5th level slot becomes 5th level. A spell that is 5th level cast with 9th level slot becomes 9th level.

The overchannel feature only works on 5th level spells -- that can be lower spells cast with 5th level slot, or can be a 5th level spell. Any spell that is 6th level or higher, whether normally or from using higher slots, is not eligible for Overchannel.

Thanks