upland vs highland

upland and highland both are nouns.

Word	Noun	Adjective	Verb	Adverb
upland	Yes	Yes	No	No
highland	Yes	Yes	No	No

As adjectives, upland and highland are synonyms defined as:

As nouns, upland and highland are synonyms defined as:

upland (noun)	highland (noun)
elevated (e.g., mountainous) land	elevated (e.g., mountainous) land

upland (adjective)	highland (adjective)
used of high or hilly country	used of high or hilly country