The only way to convert an automatic transmission to a manual transmission is to completely replace the automatic transmission assembly of the car with a manual transmission. This procedure may be difficult or impossible on some vehicles depending on the model and design.
Traditional automatic transmissions are very different in design than manual transmissions, so there is no way to convert an existing automatic transmission to a manual transmission without replacement. Major differences include the lack of a manual clutch and shifter linkage in an automatic transmission, as well as the presence of a torque converter to control shifting that is not part of a manual transmission. While some modern cars use so-called automated manual transmissions, these transmissions are also mechanically dissimilar to a classic manual transmission. Automated manual transmissions also lack a manual clutch and shifter, instead typically using one or more computer-controlled clutch packs that move to the appropriate gear via an electromechanical mechanism during shifting.
Some cars are designed in a way that makes replacing the transmission relatively straightforward, such as many rear-wheel drive sports cars. Replacing the transmission in front-wheel or all-wheel drive cars is often significantly more difficult due to the compact nature of the drivetrain in the former case, or the more complex all-wheel drive system in the latter. Some cars were never designed to use a manual transmission, and it may not be possible to find a manual transmission that is compatible with those vehicles.