Circle of Stars druids can cast Guiding Bolt without using a spell slot as many times as your proficiency bonus, but it doesn't specify what level it can be casted at. Can you cast it at any level, only 1st, or am I missing something?

Thanks.

Since it doesn’t use a spell slot it is at its base level. Upcasting requires a spell slot. 

Thanks!

The rule for casting spells at higher level starts off "When a spellcaster casts a spell using a slot that is of a higher level than the spell."

No slot, no upcast (of course there are exceptions if it says it's level).