i wouldn't have a clue where to start. maybe i should buy another hard drive.
1. Backup your wallet.dat.
2. Copy your Bitcoin folder somewhere else.
3. Start bitcoin core with
-choosedatadir=1, and set it to the directory where you copied your Bitcoin folder to.
4. Wait for bitcoin core to reindex.
5. Restart core to see if anything went wrong just in case.
6. Done. You can delete the old files but it's always recommended to keep at least 1 copy of your data files in case of corruption (so you don't need to resync).
About #3. To do this on windows, use the run window (win+r shortcut) and enter
"X:\x\x\bitcoin-qt.exe" -choosedatadir=1
Change the Xs to where bitcoin-qt.exe is located.
or just view the properties of bitcoin-qt
shortcut and add
-choosedatadir=1 in the target box. But remember to remove it later or it'll ask you to choose datadir every time you open bitcoin-qt using that shortcut.