As Rickair7777 noted, The most simple and elegant fix, if you do "fix" your logbook, is a single line-entry citing the change in a positive or negative number, and a brief explanation in the comments section. This could be done for a missed entry, incorrect entry, etc; and is an easy way that does not stand out, to make a correction from time to time if you find accumulated errors in the logbook (time totals, for example). It can be done to adjust the number of approaches or landings, time in an given category, and the comments can explain why. You could cite every page number, every date, or simply state it's a correction for student landings logged.

That said, I wouldn't do anything with it. I can't imagine an soul under the sun who might review your logbook with such a skeptical eye as to question every landing you did with a student, or who would ask you which landings you actually made, touched the controls, assisted on, followed through with sweated brow with your fingers loosely encircling the yoke, or from which you violently snatched the airplane from the student (and death's doorstep) in a heroic save rivaled only Ace Ventura himself (alrighty then). Nobody's gonna do it. Not the FAA, not an airline interviewer, not a Part 135 Chief Pilot. Not even the guy sitting across the desk from you at that first Ag job (trust me on that one). Nobody will look at the total number of landings to determine if you're a viable candidate, or be suitably impressed by more or less landings that you had with a student. The interviewer will simply yawn, turn the page, and look for something more interesting. Unless you have a wild story to tell (there I was flat on my back, open jaws of death and the gates of hell and I saw my ancestors and oh gawd did I hear the voice of tinkerbelle), anybody with a scintilla of experience in this industry will look at your flight instruction as an open and closed book. You logged the time, you got outta there, and now you're here.

Poke the bear: You wanna hear about my flight instructing experience? "Nope."

If you ever do sit across the interview desk and some yahoo in a green plaid flannel shirt, manure-stained boots propped on his desk (or black leather with the undersoles polished...you know the kind) who asks you "did you really make all those landings with those students?" then please post that person here, and I'll include them as a character in a future book. They're gonna be an elf or an evil clown, though, because nobody else would ask such a question.