mkdir /upgrade
then download the tar.gz files into that directory

cd /upgrade
unzip the files
$ gunzip IRIX6.5.18_1of4.tar.gz
$ gunzip IRIX6.5.18_2of4.tar.gz
$ gunzip IRIX6.5.18_3of4.tar.gz
$ gunzip IRIX6.5.18_4of4.tar.gz

rename the files to tardist files..
$ mv IRIX6.5.18_1of4.tar IRIX6.5.18_1of4.tardist
$ mv IRIX6.5.18_1of4.tar  IRIX6.5.18_2of4.tardist
$ mv IRIX6.5.18_1of4.tar  IRIX6.5.18_3of4.tardist
$ mv  IRIX6.5.18_1of4.tar  IRIX6.5.18_4of4.tardist

$ inst 
Inst> from /upgrade/IRIX6.5.18_1of4.tardist
Inst> open /upgrade/IRIX6.5.18_2of4.tardist
Inst> open /upgrade/IRIX6.5.18_3of4.tardist
Inst> open /upgrade/IRIX6.5.18_4of4.tardist
Inst> keep *
Inst> install upgrade
Inst> conflicts
(if no conflicts go)
Inst> go