  Non-collinear spin order that breaks space inversion symmetry and allows
efficient electric-field control of magnetism makes BiFeO$_3$ a promising
candidate for applications in low-power spintronic devices. Epitaxial strain
effects have been intensively studied and exhibit significant modulation of the
magnetic order in BiFeO$_3$, but tuning its spin structure with continuously
varied uniaxial strain is still lacking up to date. Here, we apply \emph{in
situ} uniaxial strain to a freestanding BiFeO$_3$ film and use scanning NV
microscope to image the nanoscale magnetic order in real-space. The strain is
continuously increased from 0\% to 1.5\% and four images under different
strains are acquired during this period. The images show that the spin cycloid
tilts by $\sim 12.6^\circ$ when strain approaches 1.5\%. A first principle
calculation has been processed to show that the tilting is energetically
favorable under such strain. Our \emph{in situ} strain applying method in
combination with scanning NV microscope real-space imaging ability paves a new
way in studying the coupling between magnetic order and strain in BiFeO$_3$
films.
