If it is like an E30, use the same handle you would to recline, except instead of pulling up on the handle, push down on it. If there is no weight on the seat, the seat bottom will come up.
If you do this with weight on the seat, the seat bottom will go down.
I hope that this helps.
I have fixed a similar problem and it involved removing the seat and taking out the 4 gas struts for regassing. Not a difficult job just inconvenient. While I had the seat out I took the time to lubricate all moving parts .