Layout XML in third party theme

I am a beginner in Magento 2.

I installed a third party theme, in the home page, I founded this code on Layout Update XML field

<referenceContainer name="slide.showcase">
        <block class="MagentoCmsBlockBlock" name="home-slide15">
            <arguments>
                <argument name="block_id" xsi:type="string">home-slide15</argument>
            </arguments>
        </block>

I don’t want this block (home-slide15) to be displayed in the home page, so I tried to change the Block identifier “home-slide15” to my new block. and then I saved the home page. But I noticed that the code above remains as it is without any changes and the block identifier never changed. why!

Actually I am not familiar with Magento 2, I wanted to add some new block to the home page.

please help.