It does not look like this has any screws for assembly.
If it doesn't, it the metal crimped somewhere to keep the rails from moving?
If it has rusted tapped (with screws) plates, I would lay the art on it's face, put the wide side of a nail set against the inner lip of one rail, near a corner, and then hit the nail set with a hammer until the side pops off.
Make sure that you have fresh hardware that fits the frame before you try this, in order to avoid the rusty hardware.
If the frame rails are crimped, I'd either skip the project, or follow similar steps, but reframe it in a new frame.
The crimped corners of that frame were never intended to be taken apart, and certainly never intended to be put back together.
If you can MacGyver a way to manage that, well, good on you. Otherwise I side with Ylva...new frame.
That style of frame production was never intended to be easily disassembled.
Frames like that are held together by a crimping mechanism.
Those four small indents on the corner plates are "squished" into the softer aluminum frame structure. That is what is holding the frame together.
Forcing the corners apart most likely would malform the crimps in order to release, which will mean that they may not hold securely if you are able to reassemble.
The safest way to securely re-assemble would be to crimp again with NEW replacement corner plates (and the machinery required to do the crimping) which is clearly not something you have access to.
This is why someone clever designed the kind of metal frame corners much more commonly used now that use taped screw plates, for easily performing the function you are trying to do now, disassemble and re-assemble with no harm to the frame (or it's contents).
As an optional possible solution, you may be able to find the right size/shape of those corner plates (but the kind with the screws), to replace the crimped corner plates currently installed.
However, that may be like finding a needle in a haystack. You might try looking for other older frames with the kind of corners you would need at second hand and thrift stores.
Or perhaps a local framer might have some old supplies tucked away in a corner of a drawer?
Either way, it could be a colossal amount of time spent looking for a few little bits of old outdated hardware.
If you have the time, go on a treasure hunt for replacement hardware before you totally disassemble that frame.
If someone brought that to me to try to re-use, I would explain to them why it isn't really feasible and suggest what others have mentioned, a new frame.
While the intent of trying to re-use or upcycle old frames (and many other items) is commendable, the difficulty and time spent on trying to re-use some items is simply not worth it.
I suppose one could try to jury-rig some other kind of method of re-joining those corners. But it wouldn't be reliable, and I would not offer such a service myself because of the risks.
The risks of a jury-rigged solution failing and ruining the artwork far out-way any implied savings by re-using that old frame (In my opinion).
Besides, if the item was framed so long ago (as the crimping method and rust suggests), it is quite likely there are other problematic issues inside the frame package.
Such as; the photo looks like old carboard or hardboard backer/filler(full of lignin/acids/chemicals), unknown quality matt boards, out-of-date mounting methods (bad tape/glue?), etc. etc.
If this has to come apart, it would be far better for the artwork to be given a new treatment with modern updated materials and framing techniques.
Niko's answer is spot on. I know many framers have kindness and good intentions, but it never ceases to amaze me how many will attempt to spend $100 worth of their time trying to save the customer $50, only to achieve a substandard solution. I would simply explain that this is an obsolete product and sell them a nice new frame.