I understand original LCD screen supported 1024*768 and you have replaced it for new screen capable of 1400*1050.
Please bear in mind, supported resolutions will depend on the actual graphics chipset and the graphics VBIOS (video BIOS). If you are not able to obtain 1400*1050 is because the Vbios on your computer has been locked and will not display higher than 1024*768.
In this case, Intel recommends updating to the latest generic video drivers:
https://downloadcenter.intel.com/SearchResult.aspx?lang=eng&ProdId=2301
For any other type of graphics configuration at BIOS level or video BIOS you need to contact your computer manufacturer.
Allan.