Thanks for updating in! Please understand that with bundled plugins such as Rev Slider and ME calendar plugin, we are the one who will provide the updates. In your plugins section, it may notify you for the newer versions but still you won't be able to update it because you will be connecting to our Theme.co plugin repository. Please also make sure that you have validated X theme so that you can connect to the plugin repository.
If you haven't validated X yet, please do because you will receive an error connecting to the plugin server if X is not yet validated. To validate X properly, please do the following:
1] Please login to your account and assign the license (https://theme.co/apex/licenses). You need to assign the license to your site using this url: https://www.example.com/. You have to make sure that url you just assign is the same as the one you have in your settings, Settings > General > Site Address
2] Go back to X > Overview again, and insert the license again.
And please review the changes that we made on how you are going to validated your X theme because we made a big change in the process. Please check it out here: https://theme.co/apex/forum/t/setup-product-validation/55
By the way, you can check out the latest supported plugins from this article:
Hope this helps.