The Cambridge Dictionary provides the following definitions of these two terms:
a mountainous area of a country.
high areas of land.
The definitions mentioned above are almost alike; the high areas denotes the mountainous areas, while the mountainous areas denotes a high area. But, how to use them accordingly?
Highlands would typically refer to areas that are mountainous with undulating terrain of peaks and valleys, such as the Scottish Highlands.
Uplands would refer to areas that are at a height above sea level, but are not necessarily mountainous. These would be plateaus or plains.
An example of such would be the interior of South Africa which is at a height above sea level, but is largely flat. This region is referred to locally as highveld.
Both Highlands and Uplands are plateaus, but the difference between them is:
The Highland is a plateau that is high, especially because of Mountains. While the Upland is also a plateau, but not necessarily mountainous.
