Post payments for the improvements to Fixed assets and the asset’s subaccount. This will increase the acquisition cost of the asset, raising its book value.
Market value is a separate issue. How you handle that depends on local laws and accounting standards. In some jurisdictions, you must revalue assets on a periodic basic. In others, you do not, and capital gains or losses are taken up upon disposal.
Unless you have a specific reason for revaluing the asset on your Balance Sheet I’d recommend you keep them at cost. This will keep things simpler and avoids you having to review the asset value on a regular basis. Also, unless you make appropriate notes you may make mistakes in calculating the correct capital gain for tax purposes when you sell the asset.
So, don’t rush to do all these fancy entries unless you have a valid reason.