ok well try and wear shirts that cover up your elbow for a month, if you can. on days where you need to show your elbows, wear a sunscreen on them with high spf and high uva protection. make sure it’s a good one with lots of protection, or else it won’t make a difference.
i reccomend you take a normal razor and shave your elbows , that way you can shave off the darkened layer or skin. i wouldn’t do this f you have thick/dark hair around your elbows because you might get stubble. also, if you do do this, be very carefyul because it’s extremely easy to cut yourself in that area. exfoliate often so the darkened dead skin cells can be buffed away and make room for new skin.
if this really bothers you, look for a lightening/brightening cream. make sure you get a good one though, because i heard there very bad for your skin. good luck!