"Upgrading" servers (using scripts): Early Game | Bitburner - A programming-based incremental game
If you've been following the basic guide that came with the game, chances are you should already have the script to automate the purchase of x25 8GB servers.
The scripts shown in this video allows the upgrade of the servers. It is an "indirect" method, where a first script would be used to kill active scripts on existing servers, followed by deleting the servers. The second script is then used to purchase new 256GB servers, and preload them with the desired early-hack-template.script.
Modifications of these two scripts allow for future upgrade to more powerful servers (e.g. 512 GB, 1.02 TB etc.) when required.