As far as removal. Pull the trigger assembly out cocked. Be careful not to pull the trigger, as the spring guide rod will become a missile launch when the hammer moves all the way forward. I find this the easiest method to avoid searching for parts. I put my left lower palm of my hand on the hammer and cup my fingers over the spring, guide and collar. Then grasp the trigger in a similar manner in which it is pulled with my right palm over that back brace. I then push the hammer hard into my left palm and gripping the trigger hard with the right, pull the trigger. Many times when the hammer reaches all the way forward the ball pops out of the indention, and the guide, collar and spring ends up in your fingers. It helps to wear a leather glove to absorb the shock to the hand. If it is still engaged just pull back on the collar slightly and lift while putting pressure on the ball of the guide with your thumb to stop it from a mini-launch. It helps to do this in a non congested area, and close to a somewhat softer wall just in case of a getaway launch. Also, don't forget the safety glasses!!!!!!!!
For installation, I use a forked prong welded on the end of a slip joint adjustable pliers and put the other end behind the brace where the guide goes through. With the trigger assembly in a padded vise, I compress the spring guide into the spring while holding the extra length of spring in my left hand and stopping it from flexing out until the guide reaches the back brace. I do this just enough to get the ball end in the indention on the hammer, then just cock the hammer back until engaged on the sear. Install it in the receiver cocked.
Another way without having any real special tools is like this. Use the brass tube for the guide on the spring to stop it from flexing out. Getting it compressed enough to get the ball end of the guide into the hammer indention is the tough part. Once you get the hammer cocked back slightly past center it gets easier to push it back.
Go to youtube and search Browning mainspring replacement and watch the one that is Gunsmith Rod Henrickson's video. He shows how to make a tool from Channel locks. I made one,works like a charm, use it on superposed,citoris BT99 Bt100, might have to adapt it a little