How do I improve fat on the back of my thighs? (photo)

Your thighs do not have issues related to fat distribution. The contour on the back of your thigh is cellulite, which is due to connective tissue that helps stabilize anchor and control skin mobility on pressure bearing areas.
The bluish discoloration is most likely due to visibility of superficial veins possible varicosities on Caucasian skin that had reduced elasticity and thinning due to age.
Liposuction is not going to give you a smooth fine contour, and can if anything make the contour worse.
Cellulite is notoriously difficult to treat, especially with quality long-term success. Individual areas of cellulite can be improved with Subsicion or Subsicion combined with volume augmentation.
There’s no single simple procedure to improve middle-aged flabby thighs.
As unhelpful as that may sound, I think it’s better than being told otherwise, and having unnecessary procedures or procedures that end up making things worse.
Other providers may be far more optimistic, but I would insist on seeing lots and lots of before, and after pictures of previous patients, who look just like you before spending hard, earned money, or having procedures that have potential risk involved.
Best,
Mats Hagstrom, MD