It’s hard for me to tell since I’m not sure if this Our World in Data which you mentioned is an application or something else. If it’s an application, exporting its data to an Excel sheet and then modifying that sheet shouldn’t have any impact on the application data, because exporting data is a one-way operation only.
To achieve your goal you would need to either import the updated data from your update Excel (if the application has an equivalent import option, and if it support importing data in Excel format), or alternatively edit the actual data used by the application, if this is possible AND safe to do.
There are many factors at play here which might affect this type of operation, and without knowing how this application works it’s hard to provide you with advise on how to achieve this in a safe manner.
Some applications store their data in open format files, e.g. as CSV files, whereas others use proprietary formats or even databases. You’ll have to consult the application manual to learn more about how it stores data, or peek around its installation folder and/or the folder(s) where it stores user data (e.g. in Users shared folders, maybe in a dedicated folder for this app) to see how the data is being stored.
My guess is that if your application allows you to import data from external files then it should support the CVS format, which is very common. Whether simply feeding it the raw CVS file from GitHub would result in updating the pre-existing dataset (on COVID), or creating a new dataset instead, largely depends on the application, and whether the import command can be associated to a specific dataset or not.
From my experience, if an application doesn’t clearly provide instructions on how to import data from external files and different formats, then that’s because it’s not a natively supported feature, which means that tampering with the application data files might result in data corruption (especially if trying to do so when the application is running).