That's a good effort. You can do updates to several columns in a single statement.

Declare @DayInterval INT;

Set @DayInterval = Datediff(day,(select MAX(dviendtime) from PrmDeviceVisit),GetDate());

Update dbo.PrmDeviceVisit

Set DviEndTime = DATEADD(DAY,@DayInterval,DviEndTime),

DviStartTime = DATEADD(DAY,@DayInterval,DviStartTime);

I'd usually won't use this on a heavily used environment, but as I assume you're using a personal db to learn, it's fine for now, but you'll have to learn to deal with concurrency later on.