I've installed bash linux subsys on w10, but for whatever reason my bash is rooted in a directory I cannot find on my pc.
root@DESKTOP-MLRVSAC instead of localhost.
How do I fix this issue?
DESKTOP-MLRVSAC sounds like the weird PC names Windows assigns automatically to the PC when being installed.
So I would look how that PC is named on the Windows level (sorry, I don't know Windows 10, but on Windows 7 Pro you would first right-click "Computer", then choose "Properties" from the context menu; the screen which then appears shows the computer name).
If you find out that
DESKTOP-MLRVSAC is indeed the computer name on Windows level, just use the usual Windows mechanisms / dialogs to change it (in Windows 7 Pro: Open Control Panel, choose "System", then click the link "Change Settings" at the right side in the lower half of the dialog which appears. This will open a new dialog with caption "System Properties". On the tab "Computer name" which is activated by default, there is a button "Change"; clicking it will finally open another dialog where you can input the new computer name).
You might have to logout and login again, or even to restart the PC, before Microsoft bash reflects that change. But I am not sure about that; just try.