I have a mac os sierra, and would like to update to catalina or mohave how difficult is this with an older laptop

It is basically the same for any Mac. The key issue is what year and model Mac you have. That information is available by going to About This Mac under the Apple logo. Different Macs can install different operating systems. The process typically takes about an hour, more or less, to complete the download and another 45 minutes or so to complete the installation with some automatic restarts. I strongly recommend that to do a successful download, you use a wired Ethernet connection, not Wi-Fi. Wi-Fi works fine for some people, but not for others. It is too slow for these very large downloads. You will need to see which operating system your computer can accept.

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

One may consider upGrading to High Sierra first. This may better position the computer for the Drive Conversion to the newer APFS format as well as any EFI BOOT ROM required to use the newer file format.

Tips Big Sir Upgrade - verify is the computer Qualifies

Tips Monterey Upgrade - verify is the computer Qualifies

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina, Big Sur & Monterey supports only 64 bit and there is no workaround.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may required update or upGrades to 64 Bit Versions.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation