there are two ways to change valve seals - one with head in place - requires a plug adapter and compressed air in the combustion chamber to prevent the valves from falling into the cylinder and a tool to press the valves in and remove the valve collets (locks) from the stems. (if you are finicky you can also use nylon rope in the combustion chamber with the cylinder at a few degrees BTDC then turning to towards TDC to squash it locking the valves (design dependant though)
The other aka longer way is to remove the head and do the job - long and labour intensive. The total of the job depends on how well your selected workshop is equipped - and if they are going to remove the head or not. And also dependant on the engine design and its parts availability.
in any case whenever you install new valve seals - put them in a cup of warm water to soften them - and use a plastic straw to insert them onto the valve stems so the edges of the seating grooves do not rip them, if your mechanic is going to remove the head - do not let him push the valves through the installed seal - it will tear it for sure.
