The download part can be done just as a human would do it. For example, Open browser- activity with the url to the file and then read the file in an Excel Application scope. The default download location in English Windows is “C:\Users”+ Environment.UserName +"\Downloads\myExcel.xlsx". If the name of the excel is not known you can check for files ending with *.xlsx and if found open that. Directory.GetFiles(“path”, “pattern”) returns a string array of all the paths found.
Depends on the modifications you want to make. I usually prefer reading the excel- data in to a DataTable with Read Range activity. Then do the required modifications to the DataTable and write it in to a new excel file.
Sending an email is pretty straightforward. If you are using Outlook use the Send outlook mail message and in the attachments add your newly modified excel -file path.
If this was too confusing, let me know and I can try to make a sample workflow.