So the drivers seat and especially the backrest were showing signs of wear after 13 Years and 130 K miles.

You can swap the backrest over without removing the seats from the car but I decided to remove the seats completely from the car.
Allow 2 - 3 hours working slowly.

Photo below is after I swapped both seat and back cushions over.

Tools used were:-
6 MM Allen Key for the seat runners to the bodywork, four per seat.
T50 Torx for the backrest bolts, one either side per seat. These are tight.
13 MM Socket for the seat bolts, two either side per seat.
Flat head screwdrivers say 2 mm 4 mm and 6 mm.
Small hammer.
Small torch. EDC type.
Clean floor area.
Brush to clean any dust away.
Fresh white grease to re lube the moving parts. Spray can works well.
Skill, luck, blood, sweat and maybe tears.
Some help from this Fiat Forum site.
Liquid lubricant.

1, Wind backrest as far forward as posable then push seat back and undo two 6 mm Allen key bolts from the front of the seat runner lower frame.IMG_20220806_152221173.jpg
Drivers front door side, bolt with Allen key in Position.

Drivers front handbrake side, bolt. This has a latch under the foot, see below.

2, Push seat forward and from rear footwell area remove two 6 mm Allen key bolts from the rear of the seat runner lower frame.
Rear O-S outer below the B pillar, bolt.

Rear O-S inner, bolt.

Once all the bolts are out, adjust seat to middle position, and there is just one latch to unhook the seat runner lower frame from the bodywork.
This is underneath the front inner bolt foot on both seats. You need to wiggle the seat, forward and upwards at the same time.
This is the latch with the seat out. Under the foot.

This is where it latches to the car on the passenger seat inner front bolt foot.
When assembled it is under the carpet so it is not so obvious unlike in this photo.

If you are taking both seats out then remove the passenger one first via the passenger door as there is more room.
I removed the drivers seat also via the passenger door.

3, Pop off the big back adjuster knob from the side using a large flat head screwdriver behind it. It just pops off.
Then remove both grey side panels by unclipping, starting from the front. Do the outside one first. Note they are sided.
Front end clip.

Middle one, of two clip. the second one you can't see.

Rear small clip.

Rear end tab locator.

The grey cover on the backrest adjuster / seat belt buckle side needs more wiggling / bending so do this one second.

4, Undo the tight T50 Torx from both sides and then you can pull the backrest up and off. Don't lose the white plastic 'u' shape pivot guides.
The backrest will come out easier if all four of the 13 mm bolts have been loosened a bit.
Outer side T50 in centre. Note short backrest adjuster rail sticking out.

T50 to left and and one 13mm seat bolt to the right. Note the hollow tube above the 13 mm bolt is the outer end of the cowhorn parts. See below.

Second 13mm seat bolt at the front end. Note the hollow tube above the 13 mm bolt is the outer end of the cowhorn parts. See below.

Inner side with seat belt buckle. Note long backrest adjuster rail sticking out.

5, Before refitting the backrest on the other side you will want to move the back rail adjuster bar so that the longer part is sticking out on the other side.
It is held in place by star washers on both ends.
Star washer around the backrest adjuster bar.

Starting on the long side.
Using the screwdrivers and some spray grease slowly / gently lever the star washer up the adjuster bar, trying not to break it. Replace if broken.
Stop when it is in the same position away from the end as on the other side.
Use the hammer to tap the adjuster bar through to the other side, stopping when the star washer touches the big circle frame.
Turn seat over and using the 13mm socket tap the now loose star washer down the adjuster bar until it touches the big circle frame.

You can swap the backrest over without removing the seats by following parts 3, 4 and 5.

6, Undo the four 13 mm bolt, two each side and then both lower runners can be pulled off the seat.
See photos above in section 4 for their location.
There are both ends of the two silver metal cowhorn bars and the longer thinner black seat adjuster with two parts each side to pull out.

7, Clean off any dust dirt and recover any lost items in seats. ( 1 penny, 1 hair grip and one small circular magnet was my haul. )

8, Do the same to the other seat then swap over the seat and backrest and reassemble in reverse order.

When reassembling grease up any moving parts as you go.
Would suggest you recheck all parts are tight enough especially the T50 bolts.

9, Visit fridge for some pre chilled liquid lubricant for any human parts that need it.