can anyone tell me how can i increase resolution of hdri used in this scene.
You can't change the resolution of a given HDR image, you're stuck with its inherent resolution. Try looking for a higher res image (1k, 2k, 4k...). Often HDR come in different resolution to save memory, if you look where you got yourself that one you may find it on a higher res. Otherwise a solid HDR database is www.hdrihaven.com
Good luck!