The simplest way would be to run a Windows 98 Upgrade disk, if you have one. I'll assume that you have a standard Windows 98 disk, in which case you need to make sure you can get all the hardware drivers you may need for graphics, sound, network cards and so on.
Rather than type out an entire walkthrough here for you, here are a few links to web pages where everything is explained in detail:
and click here where you can download a self extracting bootdisk image with generic CD ROM drivers to create your own Windows 98 startup floppy disk.
Using the methods detailed in the links above will wipe your hard drive of everything and give you a clean install of just Windows. Any application software (Office, SmartSuite and similar) will have to be installed after Windows.
If you want to avoid this, your only option is to source a Windows 98 Upgrade CD that will bring the existing Windows 95 install up to Windows 98 while preserving your data files and programs, although you may still have to reinstall one or two programs afterwards.
If you have an specific questions, post again, but the enclosed links should get you up and running.