I have a logfoldchange file generated using DESeq2 and I want to further used it for SPIA(pathways analysis). Spia requires 2 vectors to be passed as parameters "LogFoldChange" and "ENTERZ GENE ID". In my file I have ENSEMBL identifiers and logFoldChange. Either I have to add ENTERZ gene ID in an extra column to the corresponding EMSEMBL identifier or I can convert the EMSEMBL identifiers to ENTERZ gene ID. 

But I do not know how to do it. Can somebody help?

See this section of the workflow:

http://master.bioconductor.org/help/workflows/rnaseqGene/#annotating-and-exporting-results