There is no easy answer not cut-and-dry do this or that...
How big is she?
What breed is she if you know?
Is she high-strung, fret easily or is she pretty calm and untroubled?
Does she crib or pace a fence line?
How much hay do you currently feed her and is she able to eat and chew it well or does she leave behind wads of balled hay?
What kind of hay do you feed? Brome grass hay, alfalfa, timothy????
All feed and hay are fed by weight not volume so knowing those numbers is a must...
How much exercise does the horse get?
Is she alone or have company that may make her "move her feet"?
Does she have pasture grass or is she out on your lawn grazing often and for how long?
It isn't the horse is this and she eats and is fed that...
There is a work to calories needed "formula" if you want to call it that that must be factored in...
There is also some experimentation and documentation when you make changes so you know if there is a improvement or regression in appearances...
I always take pictures and tape measurements so true documentation is seen and noted.
I also follow directions on feeding correct amounts found on feed bag backs of most every manufacturers products.
There is honestly always some "tweaking" to a horses diet as some horses are easier to keep weight on than others and watchful, observant eyes are a must.
**You also feed with a thought to the horse is supposed to weigh "XYZ" both if they are underweight or overweight..** You don't feed say for a 1000 pound horse if the horse is supposed to weight 1200 pounds and is thin...then again you don't feed amounts for a 1400 pound horse when your horses size has them weighing 1100 pounds...you feed for where the horse is supposed to be to be healthy.
Do not overlook her teeth being checked and taken care of and older horses need more often checking.
Check she is not carrying a worm-load either.
If it is abnormally cold she might do well to wear a blanket so she not need to use her fat stores trying to stay warm...horses can and do convert fat stores to energy when cold and drop weight quickly staying warm as them must to survive and thrive.
So many things must be taken into consideration to give you a answer...it isn't so simple.