(Translated from spanish)
I accidentally marked a 136GB partition which has some files in it as swap space during installation. Can I access these files? Can I change the partition type without having to reformat?
The first thing is to make sure that you don't use that partition. Unmount it straight away if you are running the installed OS. Or just turn off the PC.
Boot from a utility disk with PARTED on it and mark the partition as whatever it was before, probably EXT2/3 if it was Linux. Then mount the partition and see if you can read the data. There is a reasonable chance that most of the data will be ok.
However, when you did the installation, there is also a good chance that the installer actually formatted the partition. If this is the case, it is less likely that the data can be recovered. But you can use a partition undelete tool to try. If you've done the above, unmount, put the partition type back to swp then run the undelete partition tool & follow the instructions.
There are too many partition undelete tools to list here but I did a quick Google search which returned a load of useful links.
Recommended recovery boot OS's: