How to replace the battery in my Remington mb-4030 trimmer ?

Here you go: https://www.ifixit.com/Guide/Remington+MB4040+Battery+Replacement/37236

I've written up a complete step-by-step tutorial on how to replace the battery on a Remington MB-300 Beard trimmer: John Fixes Stuff How to Replace the Batteries in Remington MB 300 Beard...

The rechargable nickel-cadmium is hard-wired and not replaceable.

Instructions for removing the battery from the user manual:
1. Make sure your groomer is unplugged from any outlets so you don't risk an electric shock.
2. Remove side panels/grips
3. Remove the 4 housing screws
4. Pull or gently pry the clipper halves apart, exposing the batteries.
5. Remove batteries from the unit.
6. Twist batteries to break wires. Wires may also be cut.

I also own an MB-300. The battery still works good and I loved this trimmer but I need a new cutter blade for mine.

Remove the batteries and look at the number on them, then go on ebay and you can find a replacement to make up the needed battery cell pack.

Unfortunately not. The rechargeable nickel-cadmium is hard-wired and not replaceable. The battery is hazardous waste and must be removed for proper disposal. Instructions for removing the battery from the user manual:
1. Make sure your groomer is unplugged from any outlets so you don't risk an electric shock.
2. Remove side panels/grips
3. Remove the 4 housing screws
4. Pull or gently pry the clipper halves apart, exposing the batteries.
5. Remove batteries from the unit.
6. Twist batteries to break wires. Wires may also be cut.
The user manual has a toll-free phone number to call for disposal instructions in your area: 1-800-822-8837

You will find at shaveroutlet.com. See link below

http://shaveroutlet.com/shaver-replacement-parts/remington-charging-cord-for-select-mb-models-p-551.html

You will find this charger for MB-300 at shaveroutlet.com. See link below

http://shaveroutlet.com/shaver-replacement-parts/remington-charging-cord-for-select-mb-models-p-551.html

You will find a replacement charger for Remington MB-40 at shaveroutlet.com. See link below

http://shaveroutlet.com/shaver-replacement-parts/remington-charging-cord-for-select-mb-models-p-551.html

You will find the correct battery type at shaveroutlet.com. See link below

http://shaveroutlet.com/shaver-replacement-parts/batteries/shaver-battery-pack-for-most-shavers-p-211.html

Battery charger of Remington Precision MB-40 is Remington RP00043 adapter power cord replacement parts.