i'm sure there's already a DIY out there somewhere but it's a relatively simple job with a pressure-bleeder, or a slightly more annoying (though still quite easy) 2-person job without one.
PRESSURE BLEEDER METHOD:
1. buy one of
these (no guarantee that's the best price... that was just the first google result)
2. use CLEAN turkey baster to remove excess fluid from the reservoir
3. add ~3/4 of a can of fluid to the bleeder and attach it to your reservoir
4. pump it up
5. attach supplied collection hose to the caliper bleed valve screw you want to bleed (going from furthest to closest to the reservoir, so rear right, rear left, front right, front left)
6. loosen bleed valve screw (8mm open wrench i think) ~1/8 turn and watch old/dirty/bubbly brake fluid flow down the collection hose into an empty soda bottle. when it's all coming out pure, tighten the bleed screw and move to the next corner.
7. when all 4 corners are done, disconnect the pressure bleeder (use a towel in case you drip/spray any fluid), and top off the reservoir.
good to know:
-use a fluid with a different color than what's already in the car so it's easy to know when you're flowing 100% new fluid. i use ATE super blue, (and the yellow version of the same fluid from ATE to alternate colors, though i frget what the yellow one is called).
-brake fluid will F your paint in the A, so be super clean about it, and if any fluid touches any paint, wash profusely.
-it's not necessary to jack the car up and remove the wheel if you can reach the bleed valve through your wheels (OEM 135i wheels dont need to be removed... just roll the car a few inches in ether direction to get good access.)
-the brake pedal will go straight to the floor when you start up after bleeding. pump it once or twice and you should be good to go.
-check the PSI in the pressure bleader after each corner. you NEVER want to open a bleed screw with insufficient pressure in the bleeder. this coul possibly cause the fluid to move in the wrong direction, which could theoretically introduce air into your brakes.
-you can also bleed your clutch while you're at it this way, since they both share the same reservoir... the procedure is exactly the same, except you'll need to locate the clutch bleed valve under the car.
-used hydraulic fluid is HAZMAT, so bring it to a tire/auto place to discard. do not dump it.