There are two ways to do this:
Using QMP, open the Spreadsheet you want to be your new default, rename the value in cell A2 of the first sheet to be "default.csv". If the spreadsheet is not one of your own, make a copy of it first and modify your copy:
Then use the Quadstick add-on menu to download as a QMP file and follow the instructions it will give you:
To save time with setting the "sharing" of new spreadsheets each time, I put all my configuration spreadsheets in a Google Drive folder I've named "Public" and set the sharing on that folder to "Anyone with a link" and all files placed in that folder automatically are shared.
Alternative method without QMP:
Using Windows file manager, open the Quadstick's flash drive, rename the default.csv to something like zdefault.csv or default2.csv, then rename the file you want to be the new default to default.csv
Example of making mouse.csv the default.csv. Here is an existing file set:

Rename the old default to default-old:
Rename mouse.csv to default.csv:
In QMP, click on the Reload Preferences from Quadstick button and this will show:
In my case, both the old and new default files were from the same spreadsheet.