Can you ajust the stirrups whist being on a horse?

Answer 1

You sure can! If you don't know how to do it whilst on your horse there are lots of videos.

Answer 2:

Yes, stirrups can be adjusted from the saddle while riding.

1. On an English saddle you simply move your leg forward towards the shoulder of the horse and reach under the skirt where the stirrup bar is located and the buckle for the stirrup strap is resting. You can lift the leather up and the buckle will pop out allowing you to either shorten or lengthen the leathers. Once you have it where you want you simply re-buckle the leather and put your leg back in the correct position.

2. On a western saddle it is much more difficult and really should be done from the ground if at all possible. If it's not and you need to adjust while mounted you'll need to remove your foot and lift the fender up to you. Lift the stirrup leather and remove the buckle pin, slide the solid metal half of the Belvins towards you and the other portion with the pins towards the back of the stirrup. Move it to the desired length and re-pin it. After that it goes the same way as adjusting from the ground.