100% charge lasts less than 2 hours

Before you start updating anything, if this is the original battery in the system, it's over three years old.  What kind of capacity does it have left?

Start a command prompt with admin privilege and type

powercfg /batteryreport

What are the current and design capacity numbers?

There are a great number of factors that determine battery life. Here's a couple of articles that help explain the different factors and what you can do to improve/extend it:

www.dell.com/.../how-to-improve-your-dell-laptops-battery-performance

www.dell.com/.../laptop-battery-faq

From the battery-report:

Current estimate of battery life based on all observed drains since OS install
Since OS install 1:43:22 - 4:39:37

DESIGN CAPACITY              41,998 mWh
FULL CHARGE CAPACITY 15,527 mWh
CYCLE COUNT                       -

The battery is well on its way to needing to be recycled -- it has only about 36% of its design capacity remaining.

Given that the system is over three years old, that's no unexpected.

You need  a new battery.

https://www.parts-people.com/index.php?action=item&id=30293

Hello, I have watched the links again. and I am not sure whether I can still improve the battery life  (  see my  battery status in the other reply)

Yes, batteries age starting at manufacture.  The only guarantee you have of a fresh battery is a reputable seller.

i see a new battery is needed. is it a fact that batteries loses charge even if it is brand new?  if so, is there a way to know the battery I am getting is a fresh batch (ie not been sitting around for a while)?

I see.  now we know the battery needs to be replaced.  I'd like to return to my other question, which was about updating the BIOS:

'From the youtube videos it states I need to do the BIOS upgrade from the oldest to newest version, and one at a time and allow the laptop to restart after each update, can you tell me
1. Where I can find these updates?
2. Is there a easier way to install the update, other than installing than one by one dating back to 2019?'

I cannot find a battery for my laptop, so apperciate  the link, can I ask, how to decide what battery will be suitable?

As I had asked Dell and have been told need to get Techncal Support to install the battery? 

Use the part number in the link above to search locally.

Service manual linked below.  If you do not want to or cannot replace the battery yourself, any local computer shop can do the job.

https://www.dell.com/support/home/en-us/product-support/product/inspiron-15-3585-laptop/docs

I have searched on that site and no battery found for my laptop,  and Ebay is selling the battery but I do not know how to determine whether they are fresh batch battery or genuine.

Hmm. Does the link below not take you directly to the battery for your system?  It shows as in stock and ready to ship.

https://www.parts-people.com/index.php?action=item&id=30293

Rather than asking the same question over and over again, read the "compatible laptops" list on the link I posted.

Hello, will the Dell OEM Original Inspiron 14 (5481) 2-in-1 42Wh 3-cell Laptop Battery - YRDD6 work with 'Dell Inspiron 15 3585' as that is the laptop that needs a new battery 

Great tip!  I thought it would fit. Next step is continue with my search on finding one I can buy in the UK.