Hi. I am learning about degu's but I can put up a few things I did that built up my comfort level with them and won their trust.
My cage has widely spaced bars (but is degu proof) so I could put my finger in. They would sniff it and yes sometimes nip...It was usually exploratory. If it's getting bad, a gentle puff of air on their face is a real deterrant as well as making an "eek" sound and maybe saying no. If they figure out that you'll pull away if they bite, they may do it more frequently. I hear that a lot with birds too. They learn very quickly that the negative things get the best reactions, and they keep doing it.
Move slowly, offer treats and just build up your confidence. I have only been bit once and it wasn't hard. Degu's seem legitimately sweet tempered.