Basically the proper way to replace shingles in small sections is to take it all the way up to the ridge. This way its properly done and seated properly.
But, with that special roofers tool, you can sneak under the above shingle and ease it out and then re insert a new one. It takes some work and enginuity, but you can do it. You will sub roofing cement or butyl rubber caulk for nails since you cannot send in any nails.