How do I remove nested if else conditions?
I'm assuming that computer and brand are meant to be function parameters, otherwise any kind of branching would be unnecessary.
Here's something that pretty much does what you want, assuming you just want messages that describe in clear english the quantity and brand of computers, and not necessarily have the exact messages used in your examples:
public static void Message(int quantity, string brand)
{
    Console.WriteLine(&quot;You own &quot; + quantity + &quot; &quot; + brand + (quantity !== 1 ? 's' : '');
}

The actual brand and quantities are variables that are inserted into the general message. If you want the message to be longer and use more variables, then you would probably want to create substrings to later be combined, for the sake of readability:
public static void Message(int quantity, string brand)
{
    string brandPlural = brand + (quantity !== 1 ? 's' : '');

    Console.WriteLine(&quot;You own &quot; + quantity + &quot; &quot; + brandPlural;
}
```