Do it carefully, one at a time, ensuring you understand which model belongs to which PBIX file
Right-click on Dataset or Table then click on the Edit query option as illustrated below, Power query editor will be open you can see all the datasets here
Below are the steps to copy measures from one file to another Select all measures from the list you want to copy
I tried removing the annotations from measures I had copied into the PBI model, and that allowed me to save the model
The Query Editor sources are the same in the pbix files