As far as changing the oil seal hopefully someone can chime in that has actually done this on one of these bikes themselves but you should be able to get a good idea looking at the engine manual available on the Beta USA website (under Support tab) and it looks like a similar job to most bikes. Mainly, you'll need to pull the water pump, outer and inner clutch cover, clutch, and main gear to access the seal (be careful with any gaskets during separation). Light taps with a rubber mallet inward toward the motor around the perimeter of the covers helps separate them.
Here is a link to the manual:
http://www.betausa.com/content/supportA few things to note is that the 250-300RR 2T 2016 engine manual indicates:
1. “Do not to use an air hammer/impact gun” for loosening the clutch or main gear nut and refers to a specific Beta tool (code 026140010 000) to help hold these items correctly for service. This is likely so you don't damage/Brinell the bearings or damage other components with the quick impulses that an air hammer delivers.
2. “The clutch case must be in its seat to disassemble the main gear.” The clutch and holder tool is what gives the intended reaction force for loosening the nut on the main gear.
3. “The nut on the main gear must be turned clockwise” (M20x1x5 LH).
As seen on page 30 there is a bushing/spacer between the crankshaft and oil seal that can be removed exposing the seal to where you can remove the seal with something. Do not damage or scratch the surfaces of this bushing. If the bushing is magnetic you may be able to capture it more easily but it may come out with your fingers easily anyway. Lookout for an o-ring behind this spacer/bushing. There is also a woodruff key (4x5 UNI 6606) that will likely need to be removed/replaced. I usually grab them by the side faces with a pair of dikes and replace them going back together to avoid any issues. Sometimes prying lightly with a small flathead screwdriver from the groove it is seated in works best. You don't want a sloppy fit of this key with mating components. I would inspect the seal, bushing, crankshaft surface, and o-ring very carefully as they are all involved in forming the seal.
For pulling the seal my recommendation would be to use a purpose made seal puller. Since I don’t usually have all the tools I would like when doing something I usually improvise (but don't compromise) so when removing some seals I actually use an appropriate sized allen wrench or spring hook to get behind the seal and pull straight out. This can be somewhat difficult and require some working but pliers often just start to tear the seal before becoming dislodged. Pliers can be helpful too however. Prying with an allen wrench also works but be very careful to consider what you’re prying against as aluminum will deform easily and if prying off the crankshaft you could compromise the surface of the shaft which should be avoided. Use a tough plastic shim or something if you pry against anything and don't get carried away with prying. Another thing is that if you can bend part of the seal inward from where it seats it will make pulling it out much easier as it loses much of its grip on the race it is seated in but be very careful not to slip, damage, or scratch something deforming the seal this way.
When installing a new seal I usually find a clean smooth wrench socket that very closely fits the outside diameter of the seal so as to drive it in without deforming it. You'll need to find something else (Seal Driver kit?) in order to clear the crankshaft. I've actually used the flat sides of a roll of electrical tape as a seal driver and PVC pipe against the roll of tape to clear shafts before. Clean/wipe the race where the seal is to be seated, check for any burrs or sharp edges in the aluminum casing that might tear the seal going in, lubricate the seal with oil before going in and light taps while holding and driving the seal in as squarely as possible usually equate to a successfully installed seal.
If you decide to take on this task yourself you may want to order a new safety washer for the clutch and a couple of these seals in case the first attempt to install one does not go so well. Hope this helps!