Wrong key signature

Don't know why it went wrong on the import. Could you post a reply with the original ABC file attached and someone may know?

To fix, click on the key signatures at the start of each stave and Delete them. Then, I'm afraid, you'll need to click on each C# and F# and correct them - easiest way to do this is to click on each whilst holding down the [Ctrl] key and then let go of those keys and press the Down Arrow once.

When you say "clearly written in C", if there is a "K:D" in the ABC file, then it's in D regardless of what the notes themselves may imply. It's also possible the ABC file specifies the use of a transposing instrument somehow, and the converter is seeing this and therefore doing the transposition for you. if so, it's possible that putting MuseScore into Concert Pitch mode using the large button at top left would let you see the score in C. But we're just guessing until you post the score.

Generally, though, if you want to change the key signature of a piece without changing the notes, simply drag a new signature to the first measure.