SInce it is unridable I would do the following (lots of people will probably advise against this).

1. Soak it in water till thoroughly saturated.
2. Stuff something under it to push the shape back like a plastic bag full of sand or something like that.
-let dry
3. Treat with proofide several times
4. punch holes in sides and lace flaps
5. ride and tension as necessary

Keep in mind that the integrity of leather is probably compromised and even after all that it might just go right back to the old shape and/or tear at the rivits.

It is VERY important to always keep you leather saddle dry. always stuff a plastic bag under the seat. Some people let them get wet but if you want it to last for decades you will keep it dry ALWAYS. Some people wet them to break them in faster but this only shortens their lifespan.