1
If all your files are stored on your ex-girlfriend's OneDrive, the first thing you need to do is to download those files to your PC to ensure you have a safe copy of all your files stored on your PC, because when you change the Microsoft account used to log into Windows you will loose access to that OneDrive account.
2
Then open the Settings App.
Go to Accounts - Your Info.
On that window, you should see either the option to 'Switch to a Local account' or the option to 'Stop automatically logging into apps with this account'.
Select whichever of those options exist, and that will switch the user account to a local account.
You can then add the details of your own Microsoft account to the user profile.