I have connected PowerBi to my VSO Instance, its giving default agile metrics, howeever I want to have metrics like User Story Acceptance %: for each sprint how can I do it,
USA=(Number of User Stories Accepted * 100)/ Number of User Stories Completed.
It sounds like you're connecting to the content pack in the Power BI Service. Data modelling (setting types, formats, writing measures, calculated columns, defining relationships) is only available in the PBI Desktop authoring experience. You can connect to VSO with Power BI Desktop, but you won't be able to access the full content pack. You'll have to do your own data shaping with Power Query and modelling in the data model and DAX. Once you have a data model, you can share its structure with us and we can help you out with measures.