Changing lens in a dashcam is just like you would do with a action camera that use the same kind of lenses, there are plenty of guides on that on youtube.
Of course what is different is how you take the camera apart, that i have not tried with the A139 but like others it should not be too bad, a few Phillips head screws and maybe a click think here and there.
Regarding a after market lens, first of all you of course want one that work with the size of the sensor as i recall this one are 1/1.8 but check to be sure, second you of course want that lens to be made for megapixel camera, just not small pixel count like 2 megapixel, you want 8 megapixel or higher. ( nu upper limit here 16 megapixel are often seen )
Third most optimal is the lens F number, the lower this is the more light the lens let thru, so with a focus on low light you of course want that to be a low number. F1.2 will probably be some of the best you can get, if you get up near F2.0 dont bother.
The lenses are some times called S mount or just M12 ( fine thread ) lenses.
Next is the "zoom" of the lens, it seem to me that 8 mm are very popular, i do use a 12 mm lens myself on my "zoom" camera, others use a variable zoom/focus lens, but these are huge and will not work on a camera like the A139 that sit right up against the screen.
So i think ( guessing here ) a lens will have to be around 15 - 20 mm long at the MAX to assure it clear the windscreen.
Not any i can recall that have done lens swapping on a A139 style camera, so you will probably be a trail blazer here.
There is a large thread somewhere on the forum about the "zoom" / tele cameras people play with, i think there should also be some links to online stores ´that sell lenses,,,,,, almost forgot your lens of course will need to have a IR cut filter, some dont have this as they are often made for use on CCTV cameras, which have a active IR cut filter build into the socket you screw the 12 mm lens into.
If the lens you fall for dont have a IR cut filter, the sellers will most often glue one in for free if you want it, just make sure to ask for that then, it is just a tiny little disk they glue into the back of the lens using UV hardening glue.
O i just forgot a downside to putting a tele lens in a camera like the A149 is that this camera like so many others do not have side to side adjustment, so if you mount a little offset, and have a tele lens that could be a problem as you might film off to a side then due to the curvature of a windscreen.
It is just a few degrees, but when using a tele that soon add up to a large deviation.
With a regular wide angle lens a little offset dont matter much.
My zoom camera are offset to the right, but its a mobius on a suction cup / ball hinge, so very adjustable.
you can see it in this picture of a olde setup of mine from some years ago.
If you know shooting or have tried to use a scope on a rifle, you will also know that with a lot of zoom on the scope finding the target are a bit problematic, hence why hunters often dont use scopes with a lot of zoom, at least not VS the people that shoot long range.
A tele dashcam are sort of the same, you really have to be on the ball with the aim of it as it is focused on something 5-6 car lengths ahead and the Field Of Viwe are so little, so just a little off in aim and you just have half of your own lane in the footage.
I know CUZ my suction camera some times come loose, and i pretty much have to connect to the camera and adjust its aim every time i put it back on the windscreen.
Take very little of be off to a side or too high or too low.
Using your "eagle eye" along the camera body "iron sight" only get you so far.
