How to change the value of an argument in a script?

I tried the following small simple script:

#!/bin/bash
$1="bar"
echo $1

But when I run ./script foo I get error and the value is not changed:

/home/mika/script: line 2: foo=bar: command not found
foo

I know $1 is the first argument you pass to your script. But even like this I want to change its value.

You can use the builtin set:

#!/bin/bash
set -- "bar"
echo $1

Source and more about: Change a command line argument - bash.

You can't change those variables directly. You could say:

x=$1
echo $x

Then you can use $x instead

Using $1=bar will immediately substitute the first parameter for $1, and you are essentially stating "foo=bar", and it is interpreted as a command "foo", not a variable "foo";

Don't use use $1 for this, in bash, and other shells, $1 is the first argument you pass to your script:

#!/bin/bash
echo $1

If you run the above as foo.sh hello, it will print hello since that is the 1st argument. Also, you refer to a variable as var and to a variable's contents as $var. So, to get the behavior you are expecting, just use another name for your variable and don't use a $:

#!/bin/bash
var="bar"
echo $var