@IceQueen you can create instant flow, with Manual button trigger, then add Get Items action to get items from SharePoint, then add Apply to each action and provide value (list of items) from dynamic content of Get Items in the input of apply to each. Then add Update Item action inside apply to each, provide ID value from get items dynamic content, and provide value of columns to be updated.
To test first, in Get Items provide top count of 2, then test it and if it works as expected then you can run the flow without top count. Also if you want to process hundreds of records you need to turn on pagination and provide a threshold for number of records you want to process at a time, in Get Items settings.
One more thing i would suggest, if you process hundreds of records at a time, the flow will run slow, so alternatively you can provide filter query in the Get Items action to process limited number of items at a time.
