Macbook Air 2015: random shutdowns now battery issues, battery says service needed

1st mac for me, have ipad, ipod, iphones but had always used Windows, About 11 months ago it began with random shutdowns w/ power cord & plugged in & sometimes while running battery.

Researched here on resetting smc tho I couldn't find out whether the power cord should be plugged in while pressing the 4 keys to reset smc. ??

I tried it sev times plugged in then once on battery, let it fully dc the battery sev times.. Now the battery shows Service recommended. At least that's when I learned to check it, Trying to use on battery, it dc very quickly now & usually it'll be with battery showing 15% remaining, it doesn't get lower than that I don't think.

The health status of the battery shows "Service Recommended" with only 85 cycles.

Shutdowns, now battery dc quickly, all began with only 70 cycles, Had it kept with power cord connected & few days ago I plugged in usb charger for headphones but was not charging headphones, just the usb cord was plugged into laptop & noticed the normally green light on power cord was amber & it stayed like that until I unplugged usb cord & unplugged then replugged power charging magnetic cord into laptop. Nothing external is plugged in. Occasionally I charge airpods.

The nearest Apple store is 30 miles away & I can't believe I'm having such issues. I only had it 4 yrs when all this started & I use it about 2 hrs a day. Initially the battery would last most of the day, now only 2 1/2 hrs.

Do I have to replace the battery & does anyone think I can do it on my own?

Thanks,

Carolyn