Studying prepositions is admirable, but it can lead to a lot of confusion as well! I would recommend studying verbs or cases and the prepositions that go with them, rather than studying prepositions on their own. This is because a list of prepositions doesn't give much context, and can make the task of understanding when they're used seem almost insurmountable!
The prepositions you've asked about, über and um, are good examples of how prepositions without context can lead to confusion. Über and um can both mean "about," but they are used with different verbs and in different contexts.
For instance, if you "speak about" something, you can say sprechen über, but you wouldn't say "sprechen um." Likewise, if you wanted to talk about a certain topic, you would use the phrase es geht um "it's about" - you couldn't say "es geht über."
So, unfortunately, the most concrete response is "it depends" - you would need to look at the verb and/or phrasing that you're using to determine if you should use über or um.