Change single full notes into joined half notes in 3/4 bar

You don't enter notes into any frame, but into staves and measures. And whole notes won't fit a 3/4 measure, so you can't really have done that either.
What do you mean by 'join them', "tie"?

It still isn't clear what you mean. Please attach your score, otherwise all we can do is guess. But FWIW, my guess is you don't mean really mean half note, you mean eighth note - you want to turn quarter notes into eighth notes. For that, select the passage, Ctrl+X to cut, then Edit / Paste Half Duration.