In addition to knuts answer, I'd like to expand on the difference between ä and èè.
In Swiss German, some äs sound different than other äs. But we have only one letter in the Alphabet to write it (we don't use e with accents like è, é, or ê like the French do). In written form, everything becomes an ä. (or sometimes even a simple e. It boils down to personal preference there because - like knut said - there is no rules for written Swiss German.)
Some äs are more guttural, with a very low tongue (IPA [æ]), while other äs are more closer to an e sound (IPA [ə]) (comparable to "have we met"), which is done by having the tongue slightly closer to the roof of the mouth. This depends on location/dialect, but even in the same dialect both version can occur based on the word.
Although I have never seen someone use the è character in written swiss-german communication before, It is very likely an attempt to find the phonetically closest letter to describe to the reader how exactly to pronounce that ä. In this case (übernèè), it is the second option from above paragraph, and sounds most likely to be a dialect from around Zurich.
Here are some variations of how to write "übernehmen" in swiss german. They are all "correct", because there are no rules. (Writing in swiss german is fun, you can develop your very own style.)
übernää übernä übernäh übernee überne überneh