The input shaft seal can only be replaced after the gearbox has been removed. The drive shaft seals can be replaced with the gearbox installed. Replace as follows.
Disconnect the drive shaft from the gearbox.
Use a blunt screwdriver or similar tool to carefully pry the old oil seal out.
Lubricate the new seal with oil. Establish an epiploon, working edges of an epiploon should be directed inward a case of a transmission. To install the stuffing box, use a tubular mandrel of the appropriate diameter.
Install the drive shaft.