Steps To Update Theme Without Loosing Customizations

Hi,

I would like to get the steps to update video theme without loosing any of the theme customization.
1. I am not sure of the locations or all custom code added except few.
2. I have one specific customization, where in the homepage the slider in which the ideal theme has video player and a play button,instead I had got it remove with the help of a ticket and just left it as an image.
So instead of video player and a play button I only kept and image of the listing on the slider instead.Unfortunately I forgot to ask for the customization made so now facing trouble.
I would not like to loose the customization specially of the slider and update the rest of the theme.

Would appreciate any suggestions or even help provided.

Regards,