Hello 
I have a question for which I have not found an answer
Now when you buy an plugin or theme
If you make changes to it in css or html
If I update, will all the changes be erased?
Is there a solution to this problem
thank you 

Erased, no solition except using custom file for modified functionality that does not get erased

Erased, no solition except using custom file for modified functionality that does not get erased

what i understand
There is no solution
It is better not to modify any addition or theme
Right

For themes, solution is child theme.

For themes, solution is child theme.

I didn't understand what you mean

The easiest way to update modified plugins is:

1.) Download clean plugin of the version you are currently using (you can find clean old versions on Osclasspoint)

2.) Download your production plugin files (which you have modified)

3.) Check what files have you modify with WinMerge directory comparison -> now you have a list of files you have to take attention to via update.

4.) Download clean plugin of the newest version (on which you want to update)

5.) Compare new version with clean version which you are currently using and check differences between both versions (aka, check what is new with update) and add new code to your current modified files (which you know from step 3.)

6.) Update plugin via administration

7.) Override files of the freshly updated plugin with files you update in step 5.



For themes is as mentioned the best way to use child themes.

Regards

Thanks for nice tip