I have Avo suspension (not coilovers) on my car and they were 25mm lowered, supposedly, but the car sits too high at the back but not to worry as I have some spare springs, that the car was on once before which I want to try out.

So it's a 4x4 Saff and I've never changed a spring before, what do I do?

 

Loosen the rear wheel nuts, then jack it up at the back and put axel stands in where the rear beam meets the car. Take the wheels off, disconnect the bottom of the shock absorber (17mm I think) on each side. Try to take springs out. If they won't come out, take the four bolts out that hold the diff to the body and pull down. Mind the brake lines that run to the calipers as they may be taut. Then try again to get the springs out. Then put the new ones in. Put it all back. 40 mins for a novice. 12 mins if you have done it a few times.

Charlie

 

What he said^^, but I found with AVO -25mm you may not need to do as much as they are quite short for the space, I just used to lower after taking off shocks, then managed to lever out, but is a struggle.
Are you sure you want to change them, even AVO std ride height are in the air at the rear more than the front, but still drop around 3/4" despite their name, if you ever want to go 4 /5 up with boot full etc, then don't make it silly low, rearward up slightly by 1" to 1 1/2" is normal and good.
tabetha