CapEx and Depreciation

Hello. First of all sorry for my bad english. Here I would like to ask some help from you guys regarding capitalization cost and how to depreciation it. Lets say, company A have an asset with 5 years useful lives and already two years depreciated. In mid of third year, company A decided to improve the asset and estimate that assets can have another one useful lives. The question is how to calculate this depreciation cost? Should company A calculate deprecatiation from first year (total 6 years) or just 4 years? Some source of textbook or journal regard this problem would help as I tried and can't find any and I will consider to buy. Thank you.