Through a phone call, video call or any other secure means for you and your family member.
Useful details of how to keep in touch here:
Reassure them about how you are and how the people they care about are doing.
Hygiene, clothing, medication, some exercise... If you know of any meaningful activities they do, and they can do them safely at home, invite them to continue them. Talk about any routines you've had to change also.
This might mean helping with ordering food online or making sure that they have some kind of neighbourhood support available for shopping, if you live far away. Take into account their preferences and encourage some treats.
Make sure they have access to things they like. If they have access to digital media, try to share music or books or encourage them to use things like the BBC IPlayer. For Welsh language media they can use either IPlayer or S4C Clic, in addition to the usual broadcasts of course. Take time to talk about favourite shows, films, music or books. If you can, send them something to encourage any hobbies.
Here are some suggestions:
If they don't have something like WhatsApp, try to send a family photo with a message of encouragement on the back by post, if possible. Now's the time to sort out all those old photos perhaps!
Older people carry a whole heap of experience with them and many have overcome difficult situations in the past.
Many will be in similar situations and can share their experiences and concerns with each other. If they're not on social media of some sort, then encourage them to phone, maybe a couple of people every day.
Give them the information they need that will help them. By all means talk about the current situation but remember to talk about other things too.
It's good for the person and also for the rest of the family.