Volume downsizing is not supported, including manually by our team. The workaround is to create a new 2GB volume, migrate your data to it (you can use railway volume files to download and re-upload), then delete the original 5GB volume.

i tried deletiing the volume and attaching a new volume but default it is taking to 5gb

On the Hobby plan, 5GB is both the default and the maximum volume size, so new volumes will always start at 5GB. The good news is that you're only billed for the storage you actually use, not the provisioned size, so with ~556KB of data your volume cost is negligible regardless of the 5GB capacity.

but if i want to add the new project under railway ,will i be able to add storage to it since 5gb is fully allocated to thr previous project

The 5GB limit is a per-volume maximum, not a workspace-wide total. You can create volumes in new projects without any issue - each volume gets its own 5GB capacity independently, and on the Hobby plan you can have up to 10 volumes per project. You're also only billed for actual storage used, so that 5GB volume with ~556KB of data costs almost nothing.