Here is one way to do that.
Customers <- data.frame(CustomerID=1:2,ContactName=c("A","B"),
Address=c("M","N"),City=c("Y","Z"))
Customers
CustomerID ContactName Address City
1 A M Y
2 B N Z
Customers[Customers$CustomerID==1,c("ContactName","City")] <- c("Alfred Schmidt","null")
Customers
CustomerID ContactName Address City
1 Alfred Schmidt M null
2 B N Z
If this is part of a larger process, there might be a more efficient way to do it.