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Generally, improvements could extend the useful life and increase the cost of the asset. You can record the costs of improvements using various documents - you only need to associate these costs with particular fixed asset specified as costs object. Then, you can capitalize the costs of these improvements and register changes in other parameters by creating a Capital Improvements to Fixed Assets document. For details, refer to Capital Improvements to Fixed Assets.
Also, in this document you can calculate the current depreciation if needed. If the asset was in conservation during the capital improvement works, you will need to cancel the conservation once the works are completed. After that, you will be able to calculate depreciation with the revised parameters resulted from capital improvements made to the asset.
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