"Onward" vs "Onwards"

Apparently, there is a slight difference, but only in British English:

Note: In British English, 'onwards' is an adverb and 'onward' is an adjective. In American English and sometimes in formal British English, 'onward' may also be an adverb.

So, depending on where you are, the difference maybe slight.

However, they both mean the same thing practically, and most people will understand you if you use either anyways

Onwards is an adverb, onward is an adjective in American English as well, although practically everyone will understand you no matter what version you use