There is no import or export feature. Translations cannot be downloaded as a file, edited offline, and uploaded back, and there is no way to bulk-load translations you already have.
This page covers what to do instead.
Editing translations
Edit them in place on the project's Translations screen. You can correct any string, mark it verified, retranslate it, and restore an earlier version from its history. It needs a paid plan: a free project sees an upgrade prompt there instead of the editor. See Edit translations manually.
For a translator working through a language, the status filter is the practical substitute for a spreadsheet: filter to Unreviewed, correct and verify down the list, and what remains is what still needs attention.
Enforcing specific wording
If the goal is consistent terminology rather than editing individual strings, glossary rules are the better tool. A rule applies everywhere the term appears, including in text translated in future, which a one-time import would not. See Glossary rules.
Use a glossary rule for a brand name, a product name, or a term of art. Use manual editing for a sentence that reads badly.
Reading translations programmatically
The REST API can read stored strings and their translations, so you can pull them into your own tooling for review or reporting. It is read and edit per string, not a bulk file transfer. See REST API overview.
Migrating from another service
There is no way to bring existing translations across, and coming from another translation service does not change that: Universally translates your pages from scratch. The practical migration path is to let it translate, then correct anything that matters using the steps above. What does deserve attention is your URLs, since visitors and search engines already know the old ones. See Existing URLs and redirects.