At first you need a program which is made for the job. Adobe Acrobat (Not Adobe Reader) is a common one, but has premium price. Cheaper and free alternatives exist. Search for PDF editors. Check these links, too
Some freeware has many functions disabled and to get them working you must buy the full version.
PDF editing is a gamble, because the text can be curved or rasterized (=an image, not text any more). In addition PDFs can be compressed which make them virtually uneditable because any edits, even opening the file for edits breaks the internal structure and everything explodes to nonreadable jumble.
A proper PDF editor allows to insert or edit hyperlinks. They can be anywhere. Hyperlinks can also become from the original design which is printed as PDF.
Some PDF can have edit lock=ON which need special software for bypassing or the right password.
Try with several trial versions to see, do you get anything acceptable. There's available plenty of software which claim to be a working PDF editor. This is because there's demand for them (=People think that they can make minor edits by themselves, no need to pay to the designer). Unfortunately the capablities of PDF editors vary wildly.
Warning: It can be illegal to take an existing PDF, make a new version and distribute it. You must have a written permission to use other's design as a part of your own version.