Losing weight on a bike can be rather expensive and some of that weight loss will not pay dividends for the cost.
Basic items like stems and bars can be changed to lighten the bike and you may not notice a great deal of difference unless the wallet is large.
Wheels and tyres are probably the most cost effective as good quality light wheels that will also improve the "Efficiency" of the bike can be had for a sensible price. In fact they are noted as the first improvement to be made to most bikes.
But how does the bike currently ride? Does it feel slow or heavy? If it does then the cheapest option may be N+1 to get a better bike.
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How long was I in the army? Five foot seven.
Spike Milligan