You will simply need to add package column on the right side of the operator = to add the $amount to itself.
$sql = "UPDATE `table_projects` 
        SET `package1`= `package1` + " . (int)$amount . " 
        WHERE `id` = '$userid'";
Notes:
- I have added explicit typecasting to (int)on$amount. If it is going to be decimal/float value, you can use(float)instead.
- Your code is open to SQL injection related attacks. Even real_escape_stringcannot secure it completely. Please learn to use Prepared Statements
- If you are not going to use Prepared Statements, and if the id($user_id) is integer value.
I would do typecasting on it as well
$sql = "UPDATE `table_projects` 
        SET `package1`= `package1` + " . (int)$amount . " 
        WHERE `id` = '" . (int)$userid . "'";