First of all there are some risks if done incorrectly but most cards have protection from that nowdays anyway.
You want to install an overclocking program I personally use MSI afterburner and also some kind of GPU stress tester.
GPU Stress tests:
http://www.ozone3d.net/benchmarks/fur/
https://unigine.com/products/benchmarks/valley/
MSI Afterburner:
http://download.msi.com/uti_exe/vga/MSIAfterburnerSetup.zip
In the software you should see three sliders that you'll be using such as: 'Power Limit(%)','Memory Clock(MHz)' and 'Core clock(MHz)'. Firstly without doing anything run the stress test and take note of the temperatures and frames. Then on the software increase the 'Core Clock' by 20 and press 'Apply' but after every time you do, stress the GPU to make sure it's stable at the current clock speed. If it's stable continue to increase the clock speed up in intervals until you get a crash in your graphics card, and restart your computer and continue. Once you've hit a sweet spot and it doesn't crash you can start to do this with the 'Memory clock' and increase this in intervals of 10, still testing after each increase. If you feel like you want some more clock speed you can adjust the 'Power Limit' slider and slowly carry on to increase both the 'Memory clock' and 'Core clock' until again your display crashes. Once you're at a stable clock speed you can then compare you temperatures on what they was before you overclocked and see if they've increased that much.
Overclocking is a luck thing where some people get an amazing increase in clock while others don't.