How to replace a single page in a pdf using another pdf in linux?

You could use the PDF Toolkit PDFtk:

Example:

pdftk A=inA.pdf B=inB.pdf cat A1-12 B3 A14-end output out1.pdf

The output consists of the first 12 pages of inA.pdf, followed by page 3 of inB.pdf and then pages 14 until end of inA.pdf.

Many Linux distributions provide a PDFtk package you can download and install using their package manager.

You could also use qpdf which is open source:

Same example as the one in the other response:

qpdf --empty --pages inA.pdf 1-12 inB.pdf 3 inA.pdf 14-z -- out1.pdf