I have a ds18dfl hitachi drill and i want to up grade to a better chuck . do you have any advice for a upgrade?

How to Remove Drill Chuck
If there is screw in inside center of chuck remove it, it is probably left hand threads so pretend you are tightening it and it will come free. open chuck fully and insert chuck key into holes and hit it with a hammer to remove it.

Did the receipt have it listed?

Many of these have a torque clutch that is used in screwdriver mode that will slip when a certain torque is reached as set by the ring around the chuck area. If you want it to not slip, usually you set it on a symbol that looks like a twist drill.

There are always 2 different rings to the chuck grip surface, you always hold the back one and turn the front one counter clockwise.
You should be able to do this by hand.
But if it is really tight for some reason, you could use a channel lock pliers or even a pipe wrench if you had to, but both might leave some marks.

Not sure about your drill, but some chuck screw on and could be a scew that goes inside chuck that holds it tight, some chucks press on and needs locktight to hold in place.

down inside the chuck theres a big flat head screw that should do it.

most are screwed on

It twists off counter clock wise. The best way I have found to remove the chuck is to stick a screw driver into the chuck, tighten the chuck, hold the screw driver in some type of vise and run it all full speed in reverse

time to replace the battery. eventually they won't keep a full charge anymore. they have a memory. if it's 100% charged, and you use 90% or the charge then charge it, it'll only let you run 90% of it next time. you lose 10% of the charge capacity.

Replacing the chuck on a drill is simple when you know the trick. I worked at a tool rental center while attending college and learned how to repair many tools. Open the chuck enough to remove the screw that you will see inside the chuck. It will most likely be reverse thread. The chuck is threaded on the shaft so just simply close the chuck on an allen socket and impact off with air or electric impact tool. If you do not have an impact tool close the chuck on-to the largest allen wrench that will fit into the chuck.(with the long part of the allen wrench projecting out) You will need to set the drill into a vise or strap to work-bench, then strike allen with hammer. Most chucks are removed with one quick impact of the hammer, counter clockwise. Do not squeese the drill too tightly with vise, we used a piece of leather to protect the drill. Good luck. Enjoy