When a word is typed incorrectly, autocorrect will change it when you tap the space bar. However, the option provided may not be the word you want.
Instead of deleting the whole word, just tap the backspace key once to see the replacement options – including what you had previously typed – then select the word you want to use.
Add to dictionary
Find yourself being corrected a lot while typing? You might want to add some new words to the dictionary so your iPhone can recognize them.
You can do this by using the Text Replacement section in Settings > General > Keyboard. Simply tap the ‘+’ button to add a word, then type the word into the Phrase box and leave the Shortcut box empty. Tap Save in the top right and now the iPhone won’t try to correct the phrase every time it is entered.