Try homer annotate peak to annotate your peaks with hg38 annotation (GTF). Remember you should always maintain the same genome/gene/transcript annotation throughout your analysis.

As another option, you can use ChIPseeker to annotate to the hg38 genome in the exact same way you are probably using hg19 (I assume you are following the Bioconductor guide). All you need to do is download and install this reference package and use it as your txdb object.

If you did not use a UCSC reference genome, then use a GTF file to generate your own txdb annotation using the GenomicFeatures R package.