Underwood appeared live via video conference on "Good Morning America" Tuesday to share that he is feeling "so much better."
"It was definitely a little bit of a scary time but I'm back to what I would consider normal," the 28-year-old said.
He previously posted a video detailing his symptoms and urging people of all ages to stay home.
"I can't even walk up a flight of stairs without being out of breath or go to the bathroom without having to sit down because I'm exhausted," he said in the video. "The reason I'm sharing this is not to cause fear or panic but to hopefully encourage you guys to stay at home, do your part, take care of yourselves, take care of one another."
With over 160,000 people in the U.S. diagnosed with COVID-19, Underwood said he was compelled to share that video and his diagnosis to continue spreading the message that the virus can affect all ages.
"At the time there was so much reporting going on that it might not affect the younger generation, and I consider myself 28 and healthy and it knocked me on my butt," he said. "I just wanted to share that message with everybody there that you do need to take this serious -- everybody of all ages."
Now that he has recovered, he is hoping to donate his plasma.
"I'm actually working with the hospital here in Orange County right now. They're testing everything. I still have to wait a few more days where I could actually donate my blood to another human to help them fight," he said. "But I have donated it for research purposes and I'm excited to help any way I can."
He has been quarantined with his girlfriend Cassie Randolph and her family at their home in Huntington Beach, California, and says the family has been "unbelievable" in helping him.
"I sort of for a few days was left helpless and stranded and self-isolated on their third story floor, but they've been running coffee and food up with masks and gloves and they've been so helpful and it's been so nice," he said.
During Underwood's live appearance on "GMA" Tuesday, he also spoke about his new memoir, "The First Time: Finding Myself and Looking for Love on Reality TV."
"It was a way for me to write a book and get a feel that I have my life back," he said. "After reality TV -- it's so revealing -- but I even took it a step further and decided to open up the curtains fully."