How to check/uncheck a checkbox in Wordpress post

I have the following field in a Wordpress post:

enter image description here

How can I update the following PHP script to check or uncheck the checkbox field above:

$postID = '1090';
if (condition1 == true)
    update_field('field1', 1, $postID); //check it - not working
if (condition1 == false)
    update_field('field1', 0, $postID); //uncheck it - not working

I am trying to see what the var_dump looks like by this:

$get_t = get_field('field1');

If the checkbox in the post hasn't been changed yet, I get the following:


If the checkbox in the post is checked, I get the following:

array(1) { [0]=> string(1) "1" }

Then, if the checkbox in the post is unchecked, I get the following:

string(0) ""

So, I am not sure how to actually check/uncheck it in the post through the update_field() function.