Assuming it is the type that has replaceable gland packing, the first thing to do is to adjust it as 5l and hour suggests a very large gap (in packing terms) and wear is unlikely to be that severe.
Anyway to answer your question - yes. You need to be well organised with all your new packing rings ready, cut to size, at least 3 and possibly 4 will be needed. Undo the nut and slide forward. Remove the old rings, using a screw to cut into them and pull out. Have a big rag ready to staunch the flow of water while you prepare to press the new rings in. once the first one is in the water should virtually stop. You might let 20l or so in, so not too much for the pump to deal with. When they are all in push the collar and nut back down and tighten. You may have to adjust a couple of times to get it just right with no drips when stationary and the odd drip when motoring.
