According to Nokia, the latest version of firmware (the phone’s operating system) for the N95 is 20.0.015. This was released in November 2007. According to AllAboutSymbian, this was a significant update which included:

demand paging (!), so 30MB plus free RAM after booting, faster operation, N-Gage game store previews and portal stub, new camera software, integrated Search, new Welcome apps and more

You can check your firmware version by entering *#0000# on your phone and you can update using the Nokia Software Updater .

However, sometimes even if you do have the most recent firmware, the software updater can’t update the phone. This is usually because there is a version of the firmware on your phone specifically for your mobile operator. This typically has some aspect of the phone’s functionality disabled (i.e. the SIP stack).

To get around this you can download an application called Nemesis Service Suite (NSS). This is an application which allows you to change the product code of the firmware version on your phone.

I found the following list of product codes for generic (non-mobile operator altered) firmware on Nokia-N95.net along with more detailed instructions on changing the product code:
0534841 EURO1 – Sand
0534842 FRANCE
0534843 ALPS
0534844 EURO2
0534845 TURKEY
0534847 SCANDINAVIA
0534848 BALTIAN
0534849 RUSSIAN
0534850 UKRAINE
0534851 CIS, Bulgaria
0534852 EURO3
0534853 BALKANS
0534854 GREECE, CYPROS
0534857 ISRAEL
0536058 BELARUS/MOLDOVA
0534833 SINGAPORE

0536062 EURO1 – Plum
0536063 FRANCE
0536064 ALPS
0536065 EURO2
0536066 TURKEY
0536068 SCANDINAVIA
0536069 BALTIAN
0536070 RUSSIAN
0536071 UKRAINE
0536072 CIS, Bulgaria
0536073 BELARUS/MOLDOVA
0536074 EURO3
0536075 BALKANS
0536076 GREECE, CYPROS
0536079 ISRAEL

Once youhave edited the firmware’s product code, you can now run the Nokia Software Updater once more and this time it will update the firmware on your phone.

Be aware with all these hacks that there is always the possibility of bricking your phone and always back up all your info before updating your phone.