How do viruses mutate?

Viruses change all the time. That's because they copy themselves to reproduce. Think of our cells as having their own xerox machines, which the virus takes over for its own purposes. When a virus makes a copy, sometimes a random change can occur in the copy's DNA -- the double spiral-shaped molecule which acts as a manual telling our bodies how to develop and function. If enough of these changes happen over time, a new variation or strain of a virus can emerge. 

Mutations happen in two main ways. In the first, small copying errors in the virus lead to changes in the virus' surface proteins, which sit on the outside of the virus. These proteins look to attach to your cells (much like boats seeking to tie to a dock). These changes result in more closely related virus variations, like the new Covid-19 variants.

In the other way, two variations infect the same cell in a person's body and combine to form a new or "novel" virus. This often happens when a variation that only infects animals comes into contact with a human variation. This version of mutation could have created the novel coronavirus which causes Covid-19.

Our immune systems respond differently to different virus variations, and so do vaccines.

Related Questions
What do I do if I have Covid-19 now?
Adhere To CDC Guidelines If you test positive for Covid-19, the following steps -- outlined by the Centers for Disease Control and Prevention (CDC) -- will help you care for yourself and protect others: Alert close contacts. Tell anyone you have recently come into contact with that you have Covid-19, so they can take the necessary precautions and prevent further spread. If you have concerns about confidentiality, especially in the workplace, check with your employer. Many can notify others…
Where should I go for trusted and up-to-date information on Covid-19 vaccines?
The Centers for Disease Control and Prevention (CDC) has great information on the Covid-19 vaccines, including a FAQ page. Another CDC resource will direct you to state Department of Health websites. Those sites will help you determine when you can get the vaccine in your area.
I'm young and healthy. Should I get vaccinated?
While not considered to be at high risk for severe complications from Covid-19, serious cases do occur among younger, healthier people. The Centers for Disease Control and Prevention (CDC) also warns of the potential long-term health effects of Covid-19 infection, even for those who experienced mild illness.