A simple thing to try is the Appel Reaction using CBr4 and PPh3.  It should work on your system giving an inversion of stereochemistry.

Alternatively you could try PBr3 which again should give you the bromide with inversion of stereochemistry.

The possible issue you face is the since you have a tertiary alcohol any activation of that alcohol could cause carbocation formation and thus scramble your stereocentre. 

Please look at the mechanism. This might help.

http://www.organic-chemistry.org/namedreactions/appel-reaction.shtm

A tertiary alcohol will easily react with aqueous HBr to give you the product. Tertiary butyl chloride synthesis by reaction of the alcohol with aqueous HCl is textbook example.

Wayiza is correct in that HBr would substitute a tertiary alcohol however the mechanism of such a transformation goes via a carbocation and so no control of stereochemistry is possible.

Commercially, it is chlorination that is used, with bromination mainly done on a laboratory scale. Having said that, your main issue, as others have mentioned, is that your substrate has a tertiary group and these react predominantly via a carbocation (SN1) and hence you can NOT control the stereochemistry. Carrying out the Appel reaction in hexane at a relatively low temperature, may reduce carbocation formation, but as a result of both steric crowding and low temperature, you will have both an extremely slow reaction and stil have some SN1 taking place. In addition, if this is not your final product, but you intend carrying out another SN reaction to replace the Br with another group - you are going to come up against the same problem of SN1 predominating. Your options therefore are either accept this and resolve your raceamate - making the chloro rather than the bromo compund will be cheaper with minimal effect on reaction rate as tertiary halides react rapidly, or you try an alternative route. One possibility worth exploring is to use addition to the alkene: RCH2(CH3)C=CHCN using H-Nuc (Nuc being either the Br or the group you intend to replace the Br with) in the presence of a chiaral catalyst which will favour the production of your desired stereoisomer. Ideas for the way forward can be found in either Organic Synthesis by Zweifel and Nantz or Asymetric Synthesis of Natural Products by Ari Hoskinen.

I realise this may not be that helpful, but it is the best I can suggest. Good luck.

 t-Alcohol one can be brominated using   KBr & Conc..H2SO4. The procedure is given in A text book of practical Organic chemistry by A.I.Vogel as a general method for the  preparation of alkyl bromides.

Hi All thanks for all your answers.

I tried Appel bromination using CBr4/PPh3 and also tried PBr3 but both didn't work. 

Next i tried KBr+H2SO4 to generate HBr insitu. As the reaction definitely goes via SN1 way, I am not sure the formation of carbocation is taking place or not..

As I have already mentioned the groups attached to the tertiary alcohol. I understand so far is that the Nitrile group is playing some role in formation of the carbocation. As I am not controlling stereochemistry here, I really do not bother on mixture of (R) & (S). 

The electron withdrawing nitrile group will generally lend itself towards elimination of both the tertiary alcohol and the desired bromide, in addition to which the nitrile group may be moderately unstable in the presence of HBr.

You could try activating the alcohol in situ with a mild reagent such as acetic anhydride in the presence of LiBr in DMSO or DMF, maybe acetonitrile. Trifluoroacetic anhydride is also an option as the nitrile will be moderately stable under these conditions.

You could also try to transform the alcohol into a tosylate or mesylate then substitute with bromide in an SN2 reaction.

i am agree with wayiza, first convert your tert-alcohol into a tosylated or mesylated, followed by treating with HBr/Acetic acid