I understand, however you basically have two different processes running. You have your excel file processing, and you have a client browser running somewhere else. (even if you are doing both on the same computer for testing, you are connecting to anvil in-between)
If you think about it, how would you tell some random persons browser running your website code on their own computer to change something on their screen?
One easy way is to put the information in a mutually accessible place and have the persons browser check periodically for an update. (computers don’t really do well with the request “please check to see if a string changed an infinity number of times ASAP” )
I think javascript has ways that seem like they get around this, but I am pretty sure they are just obfuscating the same thing happening with a built in method. I am no javascript developer by any means so don’t quote me on that.
There are less-advanced ways to do what you want (from a learning anvil point of view), but they are not really simpler and require more python code than using what anvil has created for you with background tasks.