You can use these links to download the official firmware version that you need to upgrade or downgrade your PSP firmware.  I’ve also linked to the guides on How to install per version of the CFW.

This list will be updated as soon as the latest firmware is released.

1.xx

| 1.50 Firmware |

2.xx

| 2.00 Firmware |
| 2.01 Firmware |
| 2.50 Firmware |
| 2.60 Firmware |
| 2.70 Firmware |
| 2.71 Firmware |
| 2.80 Firmware |
| 2.81 Firmware |
| 2.82 Firmware |

3.xx

| 3.00 Firmware |
| 3.01 Firmware |
| 3.02 Firmware |
| 3.03 Firmware |
| 3.10 Firmware |
| 3.11 Firmware |
| 3.30 Firmware |
| 3.40 Firmware |
| 3.50 Firmware |
| 3.51 Firmware |
| 3.52 Firmware | How to install 3.52 M33 |
| 3.70 Firmware |
| 3.71 Firmware | How to install 3.71 M33 |
| 3.72 Firmware |
| 3.73 Firmware |
| 3.80 Firmware | How to install 3.80 M33 |
| 3.90 Firmware | How to install 3.90 M33 |
| 3.93 Firmware |
| 3.95 Firmware |

4.xx

| 4.00 Firmware |
| 4.01 Firmware |
| 4.05 Firmware |

5.xx

| 5.00 Firmware | How to install 5.00 M33 | 5.00 M33-2 | 5.00 M33-3 | 5.00 M33-4 |
| 5.01 Firmware |
| 5.02 Firmware |