If I buy a newer version of My Passport for Mac can backup my Big Sur My current Passport is 5 years old at leastIf

Welcome to Apple Support Community.

If your current external backup disk is 5 years old, it should be replaced. However, I would not buy another My Passport. They are very low quality. Consider one of these excellent options sold by Apple: 

• LaCie 2TB Mobile Drive External Hard Drive USB-C - Apple
• LaCie 2TB Rugged USB-C Portable Hard Drive - Apple

If you have an iMac (Late 2015) or earlier, or a Mac mini (Late 2014) or earlier, you will also need this cable. 

You can then follow the steps in this Apple Support article to setup your Time Machine backup: Back up your Mac with Time Machine - Apple Support. 

Jack

https://www.amazon.com/OWC-Mercury-Elite-Storage-Solution/dp/B08612LRK3/ref=sr_1_20?crid=LHWP03YCFTSE&keywords=owc%2Bexternal%2Bdrive&qid=1652673024&sprefix=own%2Bexternal%2Bdrive%2Caps%2C148&sr=8-20&th=1 This is an external drive 7200 rpm with its own power supply, customer rating 4.4 out of 5 (69 ratings).

https://www.amazon.com/Passport-Portable-External-Drive-Black/dp/B07VTFN6HM/ref=sr_1_4?crid=3GJIIRO8EYY6U&keywords=wd%2Bpassport%2Bultra%2B2tb&qid=1652673445&sprefix=wd%2Bpassport%2Bultra%2B2%2Btb%2Caps%2C155&sr=8-4&th=1 This is a bus powered external Western Digital My Passport 5400 rpm drive rating 4.6 out of 5 (30,013 ratings)

OWC products have an excellent reputation, and the suggestion to use a drive with a separate power supply (versus bus powered) is a good one also. But rather than depend on anecdotal comments in these forums, I suggest that the potential buyer base a decision on actual statistics, not just one or two person's experience with a handful of units.

My anecdotal experience: we have used about 50 of the Western Digital passport portables since 2014 in a photography business. In that time, 2 have failed, which is a 4% failure rate over 8 years.

Typical failure rates in this industry are a few percent per year. Many of the drives from different companies actually use the same type of drives inside various enclosures, and some companies' enclosures seem to have a better track record than others.

We buy drives based on the warranty offered -- a minimum of 3 years or we don't buy it. Both OWC and Western Digital offer external drives with 3 year warranties.

For a desktop computer like an iMac, for best reliability and service life, I prefer an enclosure with

• an independent power supply and
• an enclosure that holds a full-size 3.5-inch drive, not the little 2.5-inch laptop drives that "mobile" or "portable" drives like the Passport and similar models use.

Reasons:

1. Power supply: I've had mobile drives like your Passport fail when ther motors aged and demanded more power that a USB port alone can provide.
2. 3.5-inch drive: I've never had a 3.5-inch drive fail. I've had three 2.5-inch drive fail.
3. I had a lot of cheap enclosures like the Passport give up.

rkaufmann87 and den.thed recommend OWC Mercury Elite drives; we have five of them that serving three mission-critical Macs. Flawless peace of mind.

Yes, the OWCs may be more expensive that the dreck that alleged "well-known makers" design to go on sale every other weekend. However. after suffering through a lot of "name-brand" external drives and enclosures, I realize long ago that I should have asked, "What's my data worth?"

+1 about retiring the Passport and not replacing it. I have OWC Mercury Elite Pros, some are 15+ years old and still work perfectly. The drives have had to be replaced over time (takes <5 minutes to replace) but the enclosures live on. You can find them by clicking OWC Mercury Elite Pro. If you are located somewhere in Europe, you can also find them by clicking OWC Europe to find a distributor located closer to where you live.

Personally I have tried LaCie drives in the past however I have not had the luck Jack has so I cannot recommend them.

+1 if you are serious about backing up, do yourself a favor and spend a little more on a decent external drive.

Some other good choices reside at OWC.

for example > https://eshop.macsales.com/shop/owc-mercury-elite-pro