bedtools slop will increase the size of each feature in a feature file by a user-defined number of bases. While something like this could be done with an awk '{OFS="\t" print $1,$2-,$3+ }', bedtools slop will restrict the resizing to the size of the chromosome (i.e. no start < 0 and no end > chromosome size).
slopBed [OPTIONS] -i
-b Increase the BED/GFF/VCF entry by the same number base pairs in each direction. Integer.
-l The number of base pairs to subtract from the start coordinate. Integer.
-r The number of base pairs to add to the end coordinate. Integer.
-s Define -l and -r based on strand. For example. if used, -l 500 for a negative-stranded feature, it will add 500 bp to the end coordinate.
-pct Define -l and -r as a fraction of the feature’s length. E.g. if used on a 1000bp feature, -l 0.50, will add 500 bp “upstreamâ€. Default = false.
-header Print the header from the input file prior to results.