Looks like its time to spruce up the bathroom. The grout looks old but tiles look fine, so given grout is as hard as something hard, how the heck do you remove it with a view of applying fresh grout afterwards.

Thanks for all suggestions

http://www.tesco.com/direct/unibond-grout-reviver-pen-7ml/212-8122.prd?pageLevel=&skuId=212-8122&kpid=212-8122&sc_cmp=ppc_g__&gclid=CJ6bqfWI-7cCFVMQtAodmloAew
You can also get it at good builders merchants and tile shops.
We use it quite often to spruce up bathrooms after dirty tenants..

use grout removing tool. like a 3mm thick curved, file-edged blade.

Removing it is one pain in the arse.Good luck.. 😉

Oscillating mutli tool. Noisy, dusty, but super quick. I will NEVER do it by hand again.

I used a Dremel once to really do it properly..

As Troutwrestler says get one of those tools and hope you only have to do it once in your life, as its one hell of a shite job and you DO NOT want to have to do it by hand with a grout rake