I hope you are well today and thank you for your question.
You will find it in the theme page here https://themeforest.net/item/gillion-multiconcept-blogmagazine-theme/19470306#item-description__changelog and also in the theme changelog.txt file.
Thanks for your answer, I need to ask one more thing.
I still do not understand what file I should replace to do the update. Should i update Gillion and Gillion-child, or should i only replace Gillion-child?
When I download “Installable WordPress file only”, this file is to update in Gillion-child only?
Thank very much.
You have to only update Gillion theme and not Gillion child theme.
You can use following plugin to update your theme.