I'm not sure if you just want to change the dot/comma with something different, or if they're just hidden. If they're just hidden, you can force them to unhide in the table setting a value for the field "Decimal precision" in the "Style" tab.
But if you really want to change the dot/comma behavior, you must understand that, in DataStudio, dots and commas follows the user regional preferences, and there's no simple way to customize this (actually, the best you can do is to treat the field as text and force the symbol you want to use).
Notice that in USA and some other countries, dots are used for decimal separation and commas are used as thousand separator. In my country, Brazil, we use the opposite, and other symbols/configurations exists around the globe.
I prefer to use Google in US English because I don't like translated menus. However, I know that my users usually set their accounts to Brazilian Portuguese, so when they open the dashboard made by me, it'll be displayed using their personal preferences.
So if you just want to adjust the formatting to your regional preference, there's nothing you need to do. Just assure your Google account and your users are set accordingly.
But if you really want to force the appearance of a number to a specific need, as I said, you'll need to convert it to text and format by yourself.
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