Sometimes there is no one to one translation of words because of their multiple meanings. In English "about" can also mean "almost" so it will confuse you and may be you will try to say, "I will be there in about a week" --> ich werde über/um eine Woche da sein. which is certainly wrong. Similary um can also mean, "in order to" and "um 7 Uhr" means "at 7'o clock". So it is not a good idea to translate these words into English. but it is better to learn the meanings from the situations. You can check a dictionary for all possible uses of "über" and "um". For example check the examples here: http://www.duden.de/rechtschreibung/ueber_darueber_mehr etc.

I think "um" can also mean arround and "über" can also mean above without phisical contact. You have to check the verb. Sometimes it depends on the verb you are using. sich kummern implies the preposition um + acusative case. sprechen implies the preposition über + acusative case