I've looked for it myself and search results always return complicated hacks to achieve it. Adjusting the top, bottom or right and left margins (only) are not usually a straightforward task if you do not know the right package to use.

Like for example, this StackExchange (TeX) suggestion is definitely NOT recommended.

The reasons to have to modify the page margins in the middle of a document could vary; say if you need to display an almost "full-page" figure in a landscape layout, you'd surely be touching the margins and getting additional blank pages being created if you do not reduce those margins of the current page.

The simplest way I found of doing it is using the changepage package available from ctan.org. While the package provides several commands to adjust the page margins, text blocks or the entire page design, I find the below single line command extremely useful.

\changepage{〈text height〉}{〈text width〉}{〈even-side margin〉}{〈odd-side margin〉}{〈column sep.〉}{〈topmargin〉}{〈headheight〉}{〈headsep〉}{〈footskip〉

Thus, in one single line the entire page can be re-designed in the middle of your LaTeX document.

The changepage command takes nine arguments, which can either be blank or contain a value of a length. The length value is added to the current length. If a length value is not supplied then a zero value is assigned which is then added to the current length, therefore, resulting in no change. A negative value is used to reduce the current length.