Please use a ftp client software like Filezilla and connect to your ftp server. Then navigate to wp-content/themes and upload the uncompressed theme folder (abundance folder). Filezilla will ask you if you want to overwrite the old files – click yes to confirm the operation.
Please note: 1) If you modified ay theme files make a backup of the old theme files first. 2) You need to unzip the themeforest file two times. The first archive contains all files (psd,documentation, etc. too) and you need to unzip abundance.zip a second time to get the uncompressed theme folder.