They have very different meanings and are not interchangeable. It is common for native speakers to make such a spelling error, much like the case of their / there / they're, or loose / lose.
Then: is always related to time, and the order of events.
"I showered, then I got dressed" (I showered, and having completed my shower, i proceeded to get dressed).
"i showered, than I got dressed" -> it does not make sense
Than: it is a comparison word. what comes before 'than' is compared with what comes after.
"I think apples are nicer than oranges" (I much prefer apples over oranges)
"I think apples are nicer, then oranges, then bananas" (could make sense, if you were asked to provide a list of fruits, in order of their 'niceness'.
Note that in the second case, there are different meanings, and the comma separates the two clauses. So they are still not interchangeable but there are some cases where they can fit in similarly structured sentences.