If your running Openelec it already has a template config.txt You can modify
You can do this a couple of ways
1) Either putting your sd card in a Windows PC open up and modifiy the config.txt on a program like notepad++
2) leave the card in your Pi and you can log into the pi via Shh while its running to modify the config.txt
When finished hold ctrl and press x then answer y to save changes.
I prefer the 2nd way as I never turn my PC on and you can get apps on Android (juicessh) that allow you to do this.
General safe overclock for Rpi 2 is..
I don't touch sdram as it can make your Rpi 2 unstable.
Since popcornmix has improved dynamic overclocking IMHO there's no longer any need to use force turbo either.
To be honest I find the Rpi 2 pretty good with standard values, so you may not notice much difference.
I like doing it the first way peteman ..with notepad++ its just easy
arm_freq- Frequency of ARM in MHz.
core_freq -Frequency of GPU processor core in MHz. It has an impact on ARM performance since it drives L2 cache.sdram_freq -Frequency of SDRAM in MHz.
over_voltage – ARM/GPU core voltage adjust. Values above 6 are only allowed when force_turbo or current_limit_override are specified (which set the warranty bit).
force_turbo – Disables dynamic cpufreq driver and minimum settings below. Voids Warranty.
initial_turbo -Enables turbo mode from boot for the given value in seconds (up to 60) or until cpufreq sets a frequency. Default 0
arm_freq_min – Minimum value of arm_freq used for dynamic clocking.
core_freq_min – Minimum value of core_freq used for dynamic clocking.
sdram_freq_min – Minimum value of sdram_freq used for dynamic clocking.
temp_limit – Overheat protection. Sets clocks and voltages to default when the SoC reaches this Celsius value. Setting this higher than default voids warranty. Default 85
disable_splash – If set to 1, avoids the rainbow splash screen on boot.
boot_delay - Wait for x number of seconds in start.elf before loading kernel. Default 1
gpu_mem - GPU memory in megabyte. Sets the memory split between the ARM and GPU. ARM gets the remaining memory.
I also like to up gpu mem to 320
force_turbo=1 #Voids Warranty!
boot_delay=1 #helps to avoid sdcard corruption when force_turbo is enabled.
temp_limit=80 #Will throttle to default clock speed if hit.
If you want to void your warranty then yes..
Like I said popcornmix & milhouse have worked hard with the dynamic overclocking, and it works really well on that latest versions on OpenElec and OSMC, so I I were you I wouldn't be too keen to use the force turbo mode.
Really you need to experiment and try whats best for you, but you asked for general safe overclock values thats why I gave you the values before.
Here are some benchmark tests of different overclock values on various Rpi2. Linux on Flash blog: A look at Raspberry Pi 2 performance and overclocking
You can make your own mind up