Creating relationships in Power BI is as simple as it can get but to be able to edit/create relationships in Power BI with TE is something that requires enabling one feature.
To begin with open a Power BI file and then connect Tabular Editor to it and then navigate to Tools > Preferences > Features > Check allow unsupported modelling operations
Now right click on Relationships option in TOM Explorer and choose Create > Relationship
Next a dialog box will open in which you need to specify the From and To side, make sure you fill the options correctly, the first time I tried I filled Customer in From and Sales in To
Then just click OK, save the model and go back to Power BI, and you can see relationship has been created for us.
Now if you want to edit any existing relationship, just right click and choose the Edit Relationship option.
