A little off topic but absolutly neccessary to say ;-)
You are running an very old version of SQL Server that is years out of support...
Now to your question:
The easiest way would be, just adding another disk to the system (e.g. drive m:\data) and put additional data file(s) on that drive
The correct and better way - from a performance pov - might be giving each file its own drive (maybe you already have something like this)
drive e:\data => data1.mdf
drive f:\data => data2.ndf
drive g:\data => data3.ndf
drive h:\data => data4.ndf
drive I:\data => data5.ndf
drive j:\tlog => log1.ldf
So process would be : add disks, detach database, shutdown SQL service, move files to separate disks, start SQL services, attach database
(If the reply was helpful please don't forget to upvote and/or accept as answer, thank you)