How do I replace the passenger side door mirror on the Citroen xsara picasso, please

It depends if ur car has manual or electric mirror adjustment.
l am going to assume that it is electric, if it is not then follow the same procedure but u do not need to remove the door lining.

You will need a 20 torx & 30 torx screwdriver, a large flat head screwdriver & a small flat head screwdriver.

First u need to remove the door lining so that u can unplug the electrical connector for the mirror adjustment.
Do this by prising the speaker grill out from the top, it will pop out. Remove the speaker by undoing the 4 20 torx screws that hold it in place & unplugging the connector on the back of the speaker. If u look carefully u will see 2 more 20 torx screws behind the speaker that hold the door card to the door, they will be at 10 o'clock & 5 o'clock as u look at the door.
Then pop off the trim on the door handle that u use to pull the door closed. You will find a small square hole towards the bottom end of the handle on its forward side. Put a small screwdriver in that hole & lever gently to push the trim off. This will reveal 2 30 torx screws at the top & bottom of the handle. Remove these screws. Gently pop off the surround from the interior handle that u use to open the door, it is just clipped in place, & then pop off the triangular piece of trim on the inside of the mirror at the front edge of the door. Try to lever it in the middle with the window down to avoid breaking any clips.
The door lining is now only held in place by about 7 crappy plastic clips around its circumference, insert ur large screwdriver between the door lining & the metal of the door at the lower front corner where the speaker went as this is the easiest part to start from. Lever firmly & quickly outwards & the lining should start to pop off, work ur way around the lining levering it off until all the clips r out. Then pull the lining vertically upwards to disengage it from the window seal.

With the lining off u should see a grey membrane covering the door. Using a sharp stanley knife (and a lot of care) starting with the top front corner gently pull the membrane towards u & use the stanley knife to cut the sticky foam that holds the membrane to the door. Cut away until u can fold the membrane back enough to reveal the wires coming out of the mirror & down to an electrical connector. Unplug this connector, the wires may also be clipped to the door in which case unclip them.

On the inside of the mirror (revealed when u took off the triangular trim) r 3 black 20 torx screws, remove these but remember to support the mirror so that it does not fall off & damage ur paint work. If the mirror does not fall off then give it a slight tug, but those 3 screws r all that holds it on.

Feed the plug out through the hole in the door.
Fit the new mirror in the reverse order.
Hope this helps.

as von says but i'm sure the you do not have to take the door card off as the plug for the wires is in the gap behind where the mirror fixes to the door.