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Provided the parts exist for your type of gearbox, you can either change the differential ratio, every gear will be higher meaning worse accelerations or only the 5th gear, you keep good accelerations but have a bigger gap between 4th and 5th. It's up to you to deicide according to your usual trips which option will be the best.
For example, if you drive on smaller/hilly roads with speed variations, changing the rear end might be better. If your old 5th gear was good but limit, only changing this gear might force you to drive in 4th which won't have the wanted result.
If you drive mostly on the highway at constant speed, any will help.
You can expect a gain of about 60% of the gearing change. If you increase your 5th gear by 10%, you get around 6% better FE.
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