i want to migrate only Customer data not site and stores data from M1 i am using Ubertheme migration tool ?
I've never used the Ubertheme migration tool before, however, you should choose a reliable data migration tool or find Magento migration expert to help you. Because there are many differences in structures between those 2 versions of Magento so there might be some issues during the migration process.
You can easily choose what entities and additional options you want to migrate from Magento 1 to Magento 2.
In case you need, you can consult this complete tutorial video about how to upgrade to Magento 2: https://www.youtube.com/watch?v=t2yJLuU7Saw