There are two ways to download your staking history to a file. This file then can be uploaded to a tax software or can be used at your disposal.
Download your staking rewards history from the Polymesh Portal
From the Staking Page of the Polymesh Portal, you can download your staking rewards history by clicking on the Download Reward Data button at the bottom of the page.
Download your staking rewards history from the Polymesh Block Explorer
1. Open the Polymesh Subscan Explorer
2. Copy the key from your wallet (starts with 2) you are staking with, then do a search in Subscan:
The screenshot is on testnet where addresses start with 5. On mainnet, addresses begin with 2.
3. Click on the Rewards&Slash tab, then click on on [here] to view more relevant data:
4. Click on "View all" on right hand side:
5. Scroll down to the bottom of the page and click on the button "Download all data". It will download a CSV file with your staking history.
Alternatively, you can replace the following URL with the Polymesh key you are staking with, which should direct you to a similar page with a "Download all data" button at the bottom:
If you run into any issues, please open a support ticket on the Polymesh Community Portal.