basically I have a windows 8.1 64 bit but when i download a software (e.g. firefox) it´s installed automatically in 32 bit, why and how can i change it ?
95% of software still written for 32 bits because the only software that needs 64 bits are programs capable of using more than 4gb of RAM. Firefox is only available in 32 bit version, except for a special testing version.
So unless the program needs more that 4gb of RAM the authors are unlikely to update there programs. The only way you can change it is by calling/emailing/etc the company/author and asking them to produce a 64 bit version.