Can I lock a file so nobody changes it?

No. Drime has no file lock or check-out for the files in your Drive. To keep others from changing a file, control what they can do with it, as below.

Share it with Can view or Can Download instead of Can edit. For someone who already has access, open Share on the file and change their access level.

See Change or remove access and stop sharing.

The workspace owner can hide a file or folder from chosen members with Restrict access. See Manage files in a workspace and restrict access.

Earlier versions are kept: right-click the file, choose File versions and restore the one you want. See Restore a previous version of a file.

Yes, for your Vault: it locks itself or when you lock it, and opens only with your Vault Pass. See Unlock and lock your Vault.